Get ready for the Unit 1 Take-Home Exam
Unit 1: How to Fight Infection. Your take-home exam is handed out Friday, October 2 and due Monday, October 5 at the start of class. It is a printed packet you write in pen. Everything on this page comes from what we did in class, and every answer here is hidden until you open it, so you can check yourself honestly.
- Sitting one (25 minutes). Read the 7 skills below and the Tests compared table. Say each skill out loud in your own words.
- Sitting two (25 minutes). Work the flashcards until you can say every back before you open it, then take Quiz 1 and open each answer only after you have chosen.
- Sitting three (30 minutes). Take Quiz 2, do the worked math with paper and pencil, and read one study conversation, answering its practice scenarios before you show the answer.
- Then take the practice test PDF under test conditions and check it with the key at the end. Explain every miss using the skill it matches.
The big ideas of the unit
- A test result is only as trustworthy as its controls. Positive and negative controls tell you whether a run worked before you read any patient sample.
- Every test trades two kinds of error. Sensitivity (catching the sick) and specificity (clearing the healthy) usually move in opposite directions when you move the cutoff.
- Numbers need units and steps someone else can repeat. Dilutions, notebook entries and measurements must be exact enough to repeat.
- A strong match is not proof of cause. A DNA sequence that matches a known organism, a symptom that fits a disease, or a contact map line are clues, not a diagnosis.
- Unknown is not negative. A missing lab result stays "not stated" until someone tests it.
- Antibiotics select for resistance. Using them wisely (stewardship) protects everyone, and each drug must be judged against its own standard.
- Clean technique keeps results honest. Aseptic technique and safe glove removal protect you and your data.
How the take-home exam works
- The Unit 1 Test is now a take-home exam: handed out Friday, October 2, due Monday, October 5 at the start of class. Plan on doing the math by hand.
- Printed packet, written in pen, 1000 points in five sections. Section A, multiple choice, open notes (320), then part 2 on how sure you were and why one wrong choice is wrong (80), for 400. Section B, Your evidence: answer from your own class records and cite where each record is (200). Section C, BLAST walkthrough: identify a bacterium from its DNA with NCBI BLAST, step by step (150). A computer helps, and a school library computer works; with no computer, use the printed results in the packet for the same points. Section D, Draw the ELISA: draw how an ELISA turns a target into color, then explain your drawing (150). Section E, Watch my thinking: your in-class first try and its revision, what still confuses you, and your AI Use Log (100). Paper only: nothing is uploaded.
- First try in class: on Friday, October 2, the first 5 minutes go to packet page 3, START HERE IN CLASS, in pen, with no notes. Mr. Mendoza initials it before you leave. Absent that day? Do it in class on the day you return.
- Using AI: AI may explain a concept, quiz you, or define a word, and every use goes in your AI Use Log. Do not paste an exam question into AI, do not copy an AI answer, and do not let AI write any part of Sections B, C, D or E. Every student has a short talk, about 2 minutes, with Mr. Mendoza about two or three of their answers at turn-in on Monday, October 5. This is not a punishment. If you did not log AI use, or your answers do not match your class work, the talk takes longer.
- How to use this page: work the three sittings below as you prepare for Section A, then use the skills checklist and glossary to check your own explanations in Sections B to E.
- It covers what was taught from Aug 31 through Sep 30.
- Study tools: the Unit 1 Study Guide and the Unit 1 Practice Test (key at the end), both linked at the bottom of this page and in the "Unit 1 Test prep" box under Sources on each day page.
- The rest of Unit 1 (culturing and colony counts, gene transfer and superbugs, hearing loss and cochlear implants, vaccines and herd immunity, Oct 2 to Oct 13) is tested on the Unit 2 Test, along with the PCR controls taught in the Oct 23 gel lab.
Each card names a skill, what you should be able to do, and the mistake students make most. Open the worked example after you have tried it yourself.
ELISA Controls
I can read an ELISA plate and trust the results only when the positive control is colored and the negative control is clear.
Washing removes anything that did not bind; skipping a wash leaves enzyme-linked antibody that can color a well with no target. Positive control clear: a reagent or step failed; repeat. Negative control colored: background color; repeat. Only when both behave do you compare samples to the cutoff.
Worked example
You run an ELISA. The positive control is blue, the negative control is blue, and Patient A is blue. What do you do? Answer: You cannot trust the results because the negative control shows background color. You must repeat the test.
Common mistake: Trying to interpret patient results when the negative control has color (skip reading it, the run is invalid).
Sensitivity and Specificity
I can calculate sensitivity from a 2x2 table and explain that moving the cutoff down increases sensitivity but decreases specificity.
Sensitivity = true positives / everyone who has the infection. Specificity = true negatives / everyone who does not. Lower the cutoff: sensitivity up, specificity down. Raise it: the opposite.
Worked example
Out of 100 sick people, a test catches 90. Sensitivity = 90 / 100 = 90%. If you lower the cutoff to catch more sick people, what happens? Answer: Sensitivity goes up, but specificity goes down (more false positives).
Common mistake: Confusing false positives with false negatives, or thinking you can increase both sensitivity and specificity just by moving the cutoff.
Serial Dilution Math
I can calculate serial dilution amounts, such as making a 1:10 dilution by adding 1 part sample to 9 parts diluent.
1:10 = 1 part sample in 10 parts total (1 + 9 diluent). Each 1:10 step divides the concentration by 10. Volumes: sample = final volume / 10; diluent = the rest. Traps: dividing by 9; adding 10 parts diluent (that is 1:11).
Worked example
You need 50 mL of a 1:10 dilution. How much sample and diluent do you use? Answer: 50 / 10 = 5 mL sample. The rest is diluent (50 - 5 = 45 mL diluent).
Common mistake: Adding 10 parts of diluent instead of 9, making it a 1:11 dilution, or dividing by 9 instead of 10 for the sample volume.
DNA and BLAST
I can pair DNA bases correctly (A-T, C-G) and evaluate a BLAST result using query coverage, percent identity, and E-value.
A pairs with T, C with G; no U in DNA. Judge a hit with query coverage, percent identity and E-value together; a 100% identity hit over a tiny piece is weak. A strong hit shows similarity, not cause.
Worked example
A sequence has "ATCG". The matching strand is "TAGC". A BLAST hit shows 100% identity but only 15% query coverage. Is it a strong match? Answer: No, because it only matched a tiny piece of the query sequence.
Common mistake: Assuming a 100% identity automatically means a perfect match without checking query coverage, or assuming a match proves cause.
Outbreaks and Contact Maps
I can accurately record outbreak data and avoid guessing when a result is missing.
A missing result is unknown, not negative. Symptoms never stand in for a lab result. A contact map line records a stated connection; it cannot show who infected whom or the source.
Worked example
A patient is coughing but hasn't been tested. Do you mark them positive for the disease? Answer: No, a missing result stays "unknown", and symptoms are not lab results.
Common mistake: Treating "not tested" or missing data as a "negative" result.
Notebook Entries and Gloves
I can write repeatable notebook steps with units and remove gloves safely without contaminating myself.
Exact amounts with units, order of steps, handling details. Glove removal: pinch the outside of one glove at the wrist, peel inside-out, hold it in the gloved hand, slide a bare finger inside the second glove and peel it over the first, dispose, wash hands.
Worked example
How do you take off the first glove? Answer: Pinch the outside of the glove at the wrist and peel it inside-out.
Common mistake: Writing vague instructions like "add some water" instead of exact amounts with units, or snapping gloves off and touching the outside.
Antibiotic Disk Plates
I can read zones of inhibition and compare them to specific cutoffs, understanding that a smaller zone for the same drug means less susceptible.
The zone of inhibition is the clear ring where bacteria did not grow. Same drug: a smaller zone points to a higher MIC (less susceptible). Different drugs: never rank by zone size; read each drug's own chart.
Worked example
Drug X has a zone of 15 mm. Drug Y has a zone of 20 mm. Which is better? Answer: You cannot rank different drugs by zone size. You must read each drug's own chart.
Common mistake: Comparing the zone sizes of different antibiotics to each other to see which is "strongest."
Read across one row to see how a test works and where its claim must stop. Read down the last column to see who runs it for a living.
| The test | What it looks for | How it works | The signal you see | What positive and negative mean | What it cannot tell you | Who uses it at work |
|---|---|---|---|---|---|---|
| ELISA | Specific antibodies (or antigens) | Antigen coats well, sample antibodies bind, wash, enzyme-linked secondary antibody binds, wash, substrate added | Color change, read against the controls and a cutoff (in our format, more color = more bound target; not every ELISA works that way) | Positive: signal above the cutoff, target likely present (false positives happen). Negative: below the cutoff, not detected (does not prove absence) | False positives/negatives; antibodies take time to appear and can remain after an infection or vaccination, so a positive can mean current or past infection | Medical laboratory scientist |
| DNA Sequencing & BLAST | Order of DNA bases (A, T, C, G) | Reads DNA, compares to large database of known sequences | Closest matches (hits) with Query Coverage, % Identity, E-value | Positive: Strong match. Negative: No good match | A strong match shows similarity, not proof of cause | Bioinformatics scientist |
| Disk Diffusion | Antibiotic susceptibility | Bacteria grown on agar, antibiotic disks placed, drug spreads | Zone of inhibition (clear ring where bacteria didn't grow) | Susceptible: zone at or above that drug's own susceptible cutoff. Resistant: zone below that drug's resistant cutoff. Size alone never decides; read the chart for that drug | Cannot compare zone sizes of different drugs directly | Microbiologist |
| Serial Dilution | Concentration of a sample | Step-by-step weakening by a known factor (e.g., 1:10) | Measurable, readable concentration | N/A (Yields a readable number) | Must be exact; errors multiply in steps | Epidemiologist, lab scientist |
D1. ELISA (enzyme-linked immunosorbent assay)
- Looks for: a specific antibody (or antigen) in a patient sample, such as antibodies made against an infection.
- How it works (antibody test): plastic wells are coated with the germ's antigen; blocking solution covers the bare plastic; the patient sample goes in and any matching antibodies stick; wash; an enzyme-linked second antibody that binds human antibodies goes in; wash; a colorless substrate is added and the enzyme turns it into a colored product.
- Signal: color. In the format we used, more bound antibody gave more color, but that is a rule of the format, not of every ELISA. Real labs read it with a plate reader and compare to a cutoff.
- Controls: positive control (known to contain the antibody) must show color; negative control (known to lack it) must stay clear.
- Limits: false positives and false negatives; antibodies take time to appear and can remain after an infection, so a positive can mean current or past infection.
D2. Sensitivity and specificity
- Built from a 2 x 2 table of test result (positive or negative) against true status (has it or does not).
- True positive, false positive, true negative, false negative. A false negative misses a sick person; a false positive labels a healthy one.
- A single class run cannot measure sensitivity or specificity; you need many people whose true status is known.
D3. Serial dilution
- A step-by-step way to make a sample weaker by a known factor, used to find how much of something is present and to bring strong samples into a readable range.
D4. DNA sequencing and BLAST
- Sequencing reads the order of bases (A, T, C, G) in a DNA sample. BLAST compares that read with a huge database of known sequences and reports the closest matches.
- Read a hit with query coverage, percent identity and E-value together.
D5. Disk diffusion (antibiotic susceptibility testing)
- How it works (full lab): a standardized amount of bacteria is spread evenly over an agar plate (a shallow dish of nutrient gel) to make a lawn; paper disks soaked in antibiotics are placed on top; the plate incubates about a day; the drug spreads out from each disk.
- Signal: a clear zone of inhibition around a disk where the bacteria could not grow.
- Reading it: measure the zone diameter in millimeters and compare it with that drug's own published cutoffs (susceptible, intermediate, resistant).
- MIC (minimum inhibitory concentration): the lowest concentration of a drug that stops visible growth, measured with a dilution test. A smaller zone suggests a higher MIC for that same drug.
D6. Aseptic technique
- Working so that only the microbe you mean to grow gets onto the plate or into the tube: disinfect the bench, sterilize the loop (flame or bead sterilizer), open lids briefly and at an angle, keep plates closed, never talk over open plates, dispose of cultures safely.
D7. Outbreak investigation
- Collect case records, symptoms, lab results and connections; keep observation separate from inference; revise the explanation when new evidence arrives.
- Antigen
- A molecule, often from a germ, that triggers an immune response.Example: The coating placed in the wells during our ELISA lab.
- Antibody
- A protein made by the immune system that binds to a specific antigen.Example: The primary antibodies in our patient samples.
- ELISA
- A test that uses antibodies and a color change to detect a specific antibody or antigen in a sample.Example: Testing simulated patient fluids for the outbreak.
- Well
- A small cup in a plastic plate used for testing.Example: The six-well strips we read in the ELISA lab (a clinical lab uses a 96-well plate).
- Wash
- Removing anything that did not bind in a well.Example: Rinsing out the ELISA plate between steps.
- Enzyme
- A protein that speeds up a chemical reaction.Example: The enzyme linked to the secondary antibody that turned the substrate blue.
- Substrate
- The substance an enzyme acts upon.Example: The clear liquid we added last that turned blue.
- Cutoff
- The threshold that separates a positive result from a negative one.Example: Using a specific shade of blue to decide if a patient was positive.
- Positive control
- A sample known to contain the target.Example: The well we knew should turn blue.
- Negative control
- A sample known to lack the target.Example: The well with just buffer that should stay clear.
- Background
- Color or signal where there should be none.Example: A blue negative control.
- True positive
- A sick person correctly identified as sick.Example: A sick student's well reads above the cutoff.
- False positive
- A healthy person incorrectly identified as sick.Example: A healthy student's well reads above the cutoff.
- True negative
- A healthy person correctly identified as healthy.Example: A clear ELISA well for an uninfected student.
- False negative
- A sick person incorrectly identified as healthy.Example: Missing a patient who was sick.
- Sensitivity
- The test's ability to correctly identify the sick.Example: Catching all the infected patients in our outbreak.
- Specificity
- The test's ability to correctly clear the healthy.Example: Not putting healthy students in quarantine.
- Dilution
- Making a liquid weaker.Example: Adding buffer to our concentrated samples.
- Diluent
- The liquid used to dilute the sample.Example: The buffer in our dilution tubes.
- Serial dilution
- A step-by-step dilution.Example: Making a 1:10, then 1:100, then 1:1000 series.
- Base pair
- Two complementary DNA bases (A-T, C-G).Example: Matching the sequence for BLAST.
- Sequence
- The order of bases in a DNA strand.Example: The DNA letters we copied into BLAST.
- BLAST
- A search tool that compares your sequence with a large database of known sequences and reports the closest matches.Example: Pasting our outbreak sequence to find the pathogen.
- Query coverage
- How much of your sequence matched the database hit.Example: Looking for a high percentage on the BLAST results.
- Percent identity
- How perfectly the matched parts align.Example: Getting a 100% match on the BLAST hit.
- E-value
- The number of matches this good you would expect by chance in the searched database. It is an expected count, not a probability or a percent. Smaller is stronger evidence of similarity.Example: A 0.0 value on our best BLAST hit.
- Outbreak
- A sudden rise in cases of a disease.Example: The cluster of sick students at the college.
- Case file
- A record of a patient's symptoms and lab results.Example: The notebook entries for each sick student.
- Contact map
- A diagram showing connections between people.Example: Drawing lines between students who hung out.
- PPE
- Personal Protective Equipment.Example: Our gloves and goggles.
- SDS
- Safety Data Sheet.Example: The hazard information for our lab chemicals.
- Aseptic technique
- Working to prevent contamination.Example: Disinfecting the bench and not talking over plates.
- Agar plate
- A petri dish with nutrient gel for growing microbes.Example: Where we spread our bacteria.
- Lawn
- Bacteria covering the entire surface of an agar plate.Example: The solid layer of growth before adding disks.
- Antibiotic disk
- A paper circle soaked in drug.Example: Placing the tiny white circles on our plates.
- Zone of inhibition
- The clear area where bacteria didn't grow.Example: Measuring the clear rings around the disks.
- MIC
- Minimum Inhibitory Concentration.Example: The lowest concentration of the drug that stopped visible growth.
- Susceptible
- Bacteria that the drug kills or stops from growing at doses a patient can take.Example: A zone that meets or exceeds that drug's own susceptible cutoff on the chart.
- Resistant
- Bacteria that can survive the drug.Example: Bacteria growing right up to the disk.
- Stewardship
- Using antibiotics wisely to prevent resistance.Example: Not asking for antibiotics for a cold.
What must the positive and negative controls show in an ELISA for you to trust the run?
Positive must show color, negative must stay clear.
If an ELISA negative control is colored, what does it mean?
Background color / contamination; you cannot trust the results and must repeat.
If you lower the cutoff for a test, what happens to sensitivity and specificity?
Sensitivity goes up, specificity goes down.
How do you calculate sensitivity?
True positives divided by everyone who has the infection (sick people).
How do you calculate specificity?
True negatives divided by everyone who does not have the infection (healthy people).
In a 1:10 dilution, how many parts are sample and how many are diluent?
1 part sample, 9 parts diluent (10 parts total).
What bases pair together in DNA?
A pairs with T, C pairs with G.
What three metrics should you use together to judge a BLAST hit?
Query coverage, percent identity, and E-value.
Does a strong BLAST match or a symptom prove a diagnosis?
No, a strong match is a clue or shows similarity, not proof of cause.
On a contact map or case file, what does a missing lab result mean?
It is "unknown" or "not stated," NOT negative.
Does a contact map line show who infected whom?
No, it only records a stated connection.
What is the correct way to remove the first glove?
Pinch the outside at the wrist and peel it inside-out.
What does a zone of inhibition tell you?
It is the clear ring where bacteria could not grow around an antibiotic disk.
If comparing two plates for the SAME drug, what does a smaller zone of inhibition mean?
It points to a higher MIC (the bacteria is less susceptible / more resistant).
Can you rank different drugs just by looking at their zone sizes on a plate?
No, you must read each drug's own published cutoff chart.
What is aseptic technique?
Working so that only the microbe you mean to grow gets onto the plate (e.g., keeping plates closed, not talking over them).
Choose your answer, then open it. The answer names the skill it checks and shows the working when there is math, so a miss tells you what to review.
1. You are running an ELISA test. At the end of the procedure, both the positive control well and the negative control well remain completely clear (no color). What is the correct interpretation?
- AThe positive control failed; repeat the test before reading anything.
- BThe test shows background contamination in every well; repeat it.
- CThe patient samples are definitely negative for the antibody.
- DThe test worked perfectly, so go ahead and read all of the patient samples.
Show the answer
Answer: A. (Skill 1)
2. A diagnostic test correctly identifies 95 out of 100 sick patients, but it also falsely flags 20 out of 100 healthy people as sick. If the clinic decides to raise the cutoff to reduce the false positives, what will likely happen to the test's performance?
- ASensitivity will go down, and specificity will go down.
- BSensitivity will go down, but specificity will go up.
- CSensitivity will go up, but specificity will go down.
- DSensitivity will go up, and specificity will go up.
Show the answer
Answer: B. (Skill 2)
3. You need to prepare 20 mL of a 1:10 dilution of a patient's serum. How much serum and how much diluent (buffer) should you use?
- A1 mL serum and 19 mL diluent
- B2 mL serum and 10 mL diluent
- C2 mL serum and 20 mL diluent
- D2 mL serum and 18 mL diluent
Show the answer
Answer: D. (Skill 3)
4. A BLAST search returns a hit with 100% percent identity, but the query coverage is only 8%. What is the best conclusion?
- AA perfect match that settles which pathogen is in the sample.
- BA weak match: only a tiny portion of the query aligned.
- CThe E-value must be very high, which means the match is strong.
- DThe database is broken, so you should sequence the DNA again.
Show the answer
Answer: B. (Skill 4)
5. While building a contact map for an outbreak, you note that Patient A hung out with Patient B. Patient A has tested positive, but Patient B has a cough and hasn't been tested yet. How do you record Patient B's lab status?
- ANegative, because Patient B has not tested positive so far.
- BPositive, because Patient B has a cough and was near Patient A.
- CPositive, because a contact map line proves that infection passed between them.
- DUnknown: a missing lab result stays "not stated" until it is tested.
Show the answer
Answer: D. (Skill 5)
6. Which of the following describes the correct aseptic technique for removing your first contaminated glove?
- APinch the outside of the glove at the wrist and peel it inside-out.
- BWash your gloved hands with soap before pulling them off by the fingers.
- CGrab the fingertips and pull the glove off straight.
- DSlide a bare finger inside the wrist of the glove and pop it off.
Show the answer
Answer: A. (Skill 6)
7. You are testing a new strain of bacteria against two different antibiotics on an agar plate. Drug A produces a 12 mm zone of inhibition, and Drug B produces a 20 mm zone. What is the correct way to interpret this?
- ADo not rank drugs by zone size; read each drug's own cutoff chart.
- BDrug A has the lower MIC, so it is the better choice for this patient.
- CDrug B is the stronger antibiotic because its zone is wider.
- DThe bacteria are less susceptible to Drug A than to Drug B.
Show the answer
Answer: A. (Skill 7)
Choose your answer, then open it. The answer names the skill it checks and shows the working when there is math, so a miss tells you what to review.
1. A researcher is evaluating a rapid test for a novel virus in a population of 400 people. They know from a gold-standard PCR test that 150 people actually have the virus, and 250 do not. The rapid test flags 120 of the sick people as positive, but it also flags 25 of the healthy people as positive. Calculate the sensitivity and specificity of the rapid test.
- ASensitivity = 75%, Specificity = 85%
- BSensitivity = 80%, Specificity = 10%
- CSensitivity = 80%, Specificity = 90%
- DSensitivity = 30%, Specificity = 90%
Show the answer
Answer: C. (Skill 2: Calculate sensitivity and see what moving the cutoff does)
Total sick = 150. True positives (caught by test) = 120. Sensitivity = True Positives / Total Sick = 120 / 150 = 0.80 = 80%. Total healthy = 250. False positives = 25. True negatives (cleared by test) = 250 - 25 = 225. Specificity = True Negatives / Total Healthy = 225 / 250 = 0.90 = 90%.
2. A clinician wants to increase the sensitivity of the rapid test in Question 1 so they don't miss sick patients. They lower the cutoff. The test now correctly flags 145 sick people as positive, but it flags 60 healthy people as positive. What is the new specificity, and what is the trade-off they accepted?
- ASpecificity = 24%; they accepted more false negatives to reduce false positives.
- BSpecificity = 76%; they accepted more false negatives to reduce false positives.
- CSpecificity = 96%; they accepted more true positives to reduce false positives.
- DSpecificity = 76%; they accepted more false positives to reduce false negatives.
Show the answer
Answer: D. (Skill 2: Calculate sensitivity and see what moving the cutoff does)
Lowering the cutoff caught more sick people (145/150 = 96.7% sensitivity), meaning fewer sick people were missed (reduced false negatives). However, it flagged more healthy people (60 false positives). Total healthy = 250. New True Negatives = 250 - 60 = 190. New Specificity = 190 / 250 = 0.76 = 76%. The trade-off is accepting more false positives to achieve fewer false negatives.
3. You are preparing a serial dilution for a spectrophotometer reading. You need a final volume of 300 µL of a 1:10 dilution in tube #1. From tube #1, you will make a 1:10 dilution in tube #2. How much sample and diluent go into tube #1, and what is the total dilution in tube #2?
- A30 µL sample, 270 µL diluent; Tube #2 is 1:20
- B30 µL sample, 300 µL diluent; Tube #2 is 1:20
- C30 µL sample, 270 µL diluent; Tube #2 is 1:100
- D33 µL sample, 267 µL diluent; Tube #2 is 1:100
Show the answer
Answer: C. (Skill 3: Do serial dilution math)
For Tube #1 to be a 1:10 dilution with a final volume of 300 µL: Sample = 300 / 10 = 30 µL. Diluent = 300 - 30 = 270 µL. When you take a 1:10 dilution and dilute it 1:10 again, the dilution factors multiply: 10 x 10 = 100. Tube #2 is a 1:100 dilution.
4. You have a patient sample that is highly concentrated. You perform three consecutive 1:10 dilutions (Tube 1, Tube 2, Tube 3). If you plate 1 mL from Tube 3 and count 45 bacterial colonies, what does that mean about the concentration in Tube 3 compared to the original sample?
- ATube 3 has 30 times more bacteria than the original sample had.
- BTube 3 is 1/30th the concentration of the original sample.
- CTube 3 is 1/1000th the concentration of the original sample.
- DTube 3 is 1/300th the concentration of the original sample.
Show the answer
Answer: C. (Skill 3: Do serial dilution math)
Serial dilutions multiply. Tube 1 is 1:10. Tube 2 is 1:10 of Tube 1 (1:100 of original). Tube 3 is 1:10 of Tube 2 (1:1000 of original). Therefore, Tube 3 is 1/1000th the concentration of the original sample.
5. You sequence DNA from a throat swab and BLAST it. The top hit is Streptococcus pyogenes with 96% query coverage, 98% percent identity, and an E-value of 1e-150. The patient has a sore throat and no fever. What does the BLAST result prove about the cause of the sore throat?
- AThe 96% coverage and 98% identity together confirm the diagnosis of strep throat.
- BNothing about cause: a strong match shows similarity, and a sequence is a clue.
- CNothing, because the percent identity is not 100% and the coverage is not complete.
- DThe tiny E-value mathematically proves that this organism caused the illness.
Show the answer
Answer: B. (Skill 4)
Even a very strong BLAST hit (high coverage, high identity, a tiny E-value) only shows that the DNA sequence is highly similar to S. pyogenes. It does not prove the organism is the cause of the patient's symptoms (Skill 4). A sequence is a clue, not a diagnosis. The choice that says nothing is proved because identity is below 100% reaches the right conclusion for the wrong reason: a hit below 100% can still be strong.
Dilution Problems
Problem 1. You need 40 mL of a 1:10 dilution. How much sample and how much diluent do you use?
Show the solution
40 mL / 10 = 4 mL of sample. 40 mL - 4 mL = 36 mL of diluent.
Problem 2. You have 3 mL of sample and want to make a 1:10 dilution. How much diluent do you add, and what is the final volume?
Show the solution
3 mL x 10 = 30 mL final volume. 30 mL - 3 mL = 27 mL of diluent.
Problem 3. You need 150 µL of a 1:5 dilution. How much sample and diluent do you use?
Show the solution
150 µL / 5 = 30 µL of sample. 150 µL - 30 µL = 120 µL of diluent.
Problem 4. You add 5 mL of sample to 20 mL of diluent. What is the dilution factor?
Show the solution
Final volume = 5 + 20 = 25 mL. Dilution = 5 / 25 = 1:5.
Problem 5. You need to make 60 mL of a 1:4 dilution. How much sample and diluent?
Show the solution
60 / 4 = 15 mL sample. 60 - 15 = 45 mL diluent.
Problem 6. A student adds 1 mL of sample to 10 mL of diluent. What dilution did they make?
Show the solution
Final volume = 1 + 10 = 11 mL. The dilution is 1:11, not 1:10!
Sensitivity and Specificity Problems
Problem 7. A new test is given to 200 sick patients. It correctly catches 170 of them as positive. 30 test negative. What is the sensitivity?
Show the solution
True Positives = 170. Total sick = 200. Sensitivity = 170 / 200 = 85%.
Problem 8. Out of 500 healthy individuals, a test correctly identifies 480 as negative. 20 test positive. What is the specificity?
Show the solution
True Negatives = 480. Total healthy = 500. Specificity = 480 / 500 = 96%.
Problem 9. In a trial, 50 people have the disease and 150 do not. The test gives a positive result for 45 of the sick people. What is the sensitivity?
Show the solution
True Positives = 45. Total sick = 50. Sensitivity = 45 / 50 = 90%.
Problem 10. Using the same trial from Problem 9, the test gives a positive result for 15 of the 150 healthy people (false positives). What is the specificity?
Show the solution
False Positives = 15. True Negatives = 150 - 15 = 135. Specificity = 135 / 150 = 90%.
Problem 11. You lower the cutoff for a test. It now catches 49 out of 50 sick people, but it also flags 40 out of 150 healthy people as positive. What are the new sensitivity and specificity?
Show the solution
Sensitivity = 49 / 50 = 98%. Specificity = (150 - 40) / 150 = 110 / 150 = 73.3%.
Problem 12. A test has a sensitivity of 100% and a specificity of 50% for a population of 100 sick and 100 healthy people. How many false negatives and false positives are there?
Show the solution
100% sensitivity means it catches all 100 sick (0 false negatives). 50% specificity means it correctly clears 50 healthy, but flags the other 50 (50 false positives).
Two hosts walk through the unit and stop for practice scenarios. Answer each scenario yourself before you show the answer.
MI Unit 1 - Audio 1: Safety, Notebooks, and Outbreaks
Alex: Welcome to the Unit 1 study podcast for Mr. Mendoza's Medical Interventions class! I'm Alex.
Sam: And I'm Sam. Today we're getting into the basics: lab safety, keeping good records, and tracking an outbreak. Since we’re starting fresh, we’re going to define every science term we use.
Alex: Let’s start with the SDS, which stands for Safety Data Sheet. It’s a document that tells you the hazards of any chemical you use and how to handle it safely.
Sam: Exactly. And keeping yourself safe also means knowing how to take off your gloves, what we call Personal Protective Equipment, or PPE. To do it safely: pinch the outside of one glove at the wrist, peel it inside-out, hold it in your gloved hand, then slide a bare finger inside the second glove and peel it over the first. Dispose of them and always wash your hands.
Alex: Good practice. Now, when you're recording your work, you need repeatable notebook entries. This means writing exact amounts with units, the exact order of steps, and any handling details so someone else could repeat your work perfectly.
Sam: Speaking of recording, let's talk about outbreak case files. Remember: a missing lab result is just "unknown," not "negative." Until you test for it, you don't know.
Alex: And remember that symptoms are what the patient feels, while lab results are measurable data. Symptoms never stand in for a lab result. Finally, a contact map line only records a stated connection between two people; it cannot show who infected whom or where the disease started.
Sam: Alright, let's do three practice scenarios. We will stop after asking, so stop and think about your answer before you open it.
Alex: Scenario 1: You are looking at a patient's file. It says they have a fever and cough, but the COVID-19 test field is blank. A classmate says, "Well, they don't have COVID." What is the error here?
Think first, then show the answer
Sam: The explanation: The error is assuming a blank field means negative. An unknown result stays "unknown" until someone tests it. This is Skill 5: keeping outbreak case files honest.
Alex: Scenario 2: You draw a line on a contact map between Patient A and Patient B because they sit together at lunch. Does this line prove Patient A infected Patient B?
Think first, then show the answer
Sam: The explanation: No, it does not. A contact map line only records a stated connection. It cannot show the direction of infection or the source. That's also Skill 5.
Alex: Scenario 3: You write in your notebook: "Added some buffer to the tube and mixed it." Why is this a bad entry?
Think first, then show the answer
Alex: The explanation: It lacks exact amounts with units and specific steps. Someone else cannot repeat "some buffer." You need to write exact volumes, like "Added 10 milliliters of buffer." This is Skill 6: writing repeatable notebook entries.
Sam: Great job. Remember the 6 Rs: Record, Reduce, Review, Reflect, Recite, Revise. See you next time!
MI Unit 1 - Audio 2: The Antibody ELISA
Alex: Welcome back! Today we are tackling the ELISA. That stands for enzyme-linked immunosorbent assay. We're looking at the antibody version of this test.
Sam: Right. We use an ELISA to look for a specific antibody in a patient sample. Antibodies are proteins your body makes to fight off a specific germ. Let’s walk through the steps.
Alex: First, the plastic wells are coated with the germ's antigen, that's the target part of the germ. Then, a blocking solution covers the bare plastic so things don't stick where they shouldn't.
Sam: Next, you add the patient sample. If the patient has the matching antibodies, they stick to the antigen. Then, we wash the well. Washing removes anything that did not bind.
Alex: After the wash, we add an enzyme-linked second antibody. This one is designed to bind to human antibodies. Then, another wash. Skipping a wash is bad because it leaves the enzyme-linked antibody in the well even if it didn't bind, which can cause false color later.
Sam: Finally, we add a colorless substrate. The enzyme acts on the substrate to turn it into a colored product. In this format, more color means more bound antibody. That is a rule of this format, not of every ELISA.
Alex: But we only trust the results if the controls behave. The positive control, which we know has the antibody, MUST show color. The negative control, which we know lacks the antibody, MUST stay clear. If the positive is clear, a step failed. If the negative is colored, there's background noise. In either case, repeat the test.
Sam: Let's practice. Stop after each question and think before you open the answer.
Alex: Scenario 1: You run an ELISA. Your patient well turns blue, but your negative control also turns blue. What does this mean?
Think first, then show the answer
Sam: Explanation: This means the test is invalid due to background color. You cannot trust the patient result. You must repeat the test. This relates to Skill 1: trusting controls.
Alex: Scenario 2: You are rushing and forget the wash step after adding the enzyme-linked second antibody. What will happen when you add the substrate?
Think first, then show the answer
Sam: Explanation: The well will turn colored regardless of whether the patient has the antibody. The unbound enzyme-linked antibody wasn't washed away and will react with the substrate, causing a false positive. This is Skill 1.
Alex: Scenario 3: Your positive control stays clear, but the patient well turns blue. Can you diagnose the patient?
Think first, then show the answer
Alex: Explanation: No. If the positive control fails to show color, a reagent or step failed. You cannot trust any result on the plate and must repeat it. Skill 1 again!
Sam: That wraps up ELISA! Remember your 6 Rs: Record, Reduce, Review, Reflect, Recite, Revise.
MI Unit 1 - Audio 3: Sensitivity, Specificity, and Cutoffs
Alex: Welcome to Audio 3. Today involves some math, so grab some paper. We’re talking about sensitivity and specificity, and how we measure test errors.
Sam: Every test has limits. A false positive is when the test says you have the disease, but you're actually healthy. A false negative is when the test says you're healthy, but you actually have the disease.
Alex: False negatives harm the sick person because they miss out on treatment. False positives harm the healthy person by causing unnecessary stress and potentially harmful treatments.
Sam: We measure a test's performance using a 2 by 2 table, comparing the test result against the true status of the patient. Sensitivity is the number of true positives divided by everyone who ACTUALLY has the infection. Specificity is the true negatives divided by everyone who actually does NOT have it.
Alex: And here is the big rule: when you move the cutoff, the line deciding what counts as positive, these two move in opposite directions. Lower the cutoff: you catch more sick people (sensitivity goes up), but you falsely label more healthy people (specificity goes down). Raise it, the opposite happens.
Sam: Time for practice scenarios.
Alex: Scenario 1: Out of 100 people who truly have a disease, a test correctly identifies 90 of them as positive. It misses 10. What is the sensitivity?
Think first, then show the answer
Sam: Explanation: Sensitivity is true positives divided by total true sick. So, 90 true positives divided by 100 total sick people equals 0.90, or 90%. This is Skill 2.
Alex: Scenario 2: You have a test for a very dangerous disease where missing a diagnosis is deadly. Should you lower or raise the cutoff for a positive result, and what happens to specificity?
Think first, then show the answer
Sam: Explanation: You should lower the cutoff to catch as many sick people as possible (increasing sensitivity). However, this will decrease specificity, meaning you'll get more false positives. That's the trade-off. Skill 2.
Alex: Scenario 3: Out of 200 people who do NOT have a disease, a test gives a negative result for 150 of them, but gives 50 a positive result. What is the specificity?
Think first, then show the answer
Alex: Explanation: Specificity is true negatives divided by total true healthy. That's 150 true negatives divided by 200 true healthy people, which is 0.75, or 75%. Skill 2!
Sam: Excellent math! Close out with the 6 Rs: Record, Reduce, Review, Reflect, Recite, Revise.
MI Unit 1 - Audio 4: Dilutions and DNA BLAST
Alex: Hey everyone! Audio 4 is here. We are going over serial dilution math and reading DNA BLAST results.
Sam: Let's define serial dilution. It's a step-by-step way to make a sample weaker by a known factor. A common one is a 1:10 dilution. This means 1 part of your sample in 10 parts total.
Alex: Right, so that's 1 part sample plus 9 parts diluent, the liquid you use to thin it out. The common trap is adding 10 parts diluent to 1 part sample, which makes a 1:11 dilution! Another trap is dividing by 9. Always remember: sample volume = final volume divided by 10. The diluent is whatever is left.
Sam: Now for DNA. DNA bases pair up: Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G). Remember, there is NO Uracil (U) in DNA; that's only in RNA.
Alex: When we sequence DNA, we can use a search tool called BLAST to compare the read with a database of known sequences. We judge a match using three things: query coverage (how much of our sequence aligned), percent identity (how exactly the letters matched), and E-value (how many matches this good you would expect by chance; it is a count, not a probability; closer to zero is better).
Sam: Remember the big idea: a strong hit shows similarity, not cause. Just because a patient's sample has a bacteria's DNA doesn't prove that bacteria caused their symptoms.
Alex: Practice time!
Sam: Scenario 1: You need to make a 20 milliliter final volume of a 1:10 dilution. How much sample and how much diluent do you mix?
Think first, then show the answer
Alex: Explanation: Sample volume equals final volume divided by 10. So 20 divided by 10 is 2 mL of sample. The rest is diluent, so 20 minus 2 is 18 mL of diluent. This is Skill 3: serial dilution math.
Sam: Scenario 2: You match a DNA sequence on BLAST. The percent identity is 100%, but the query coverage is only 4%. Is this a strong hit?
Think first, then show the answer
Alex: Explanation: No, it is a weak hit. Even though the letters matched perfectly, it only matched a tiny piece (4%) of your sequence. You must judge them together. This is Skill 4.
Sam: Scenario 3: You have a DNA sequence: A-T-C-G. What is the complementary DNA strand?
Think first, then show the answer
Sam: Explanation: A pairs with T, and C pairs with G. So the complementary strand is T-A-G-C. There is no U in DNA. Skill 4 again!
Alex: You've got this! End your session with the 6 Rs: Record, Reduce, Review, Reflect, Recite, Revise.
MI Unit 1 - Audio 5: Antibiotics and Stewardship
Alex: Welcome to Audio 5. Let's talk about antibiotics and how we test them.
Sam: A big concept here is antibiotic stewardship, using antibiotics wisely to protect everyone, because using them selects for resistant bacteria. Resistance and gene transfer are coming up next in Unit 2, but for now, let's look at how we measure susceptibility.
Alex: We use disk diffusion on an agar plate, which is a shallow dish of nutrient gel. We spread bacteria evenly over it to make a "lawn." Then, we place paper disks soaked in different antibiotics on top and let it incubate.
Sam: The drug spreads out from the disk. If the drug kills the bacteria or stops them growing, it leaves a clear ring called the "zone of inhibition." We measure this zone's diameter in millimeters.
Alex: We also talk about MIC, or minimum inhibitory concentration. This is the lowest concentration of a drug that stops visible growth. When looking at the same drug on different strains, a smaller zone of inhibition suggests a higher MIC, meaning the bacteria is less susceptible to it.
Sam: But here is the critical rule: never rank different drugs by their zone size. Each drug diffuses differently. You have to read each drug's zone against its own specific chart to see if it is susceptible, intermediate, or resistant.
Alex: Let's practice.
Sam: Scenario 1: You have two plates with different bacteria. You place a Penicillin disk on both. Plate A has a 25 mm zone, Plate B has a 10 mm zone. Which bacteria has a higher MIC for Penicillin?
Think first, then show the answer
Alex: Explanation: Plate B has the higher MIC. The smaller zone of inhibition means it took a higher concentration of the drug to stop the bacteria. This is Skill 7: reading disk plates.
Sam: Scenario 2: On one plate, Drug X has a 20 mm zone and Drug Y has a 15 mm zone. Can you conclude that Drug X is the better antibiotic for this infection?
Think first, then show the answer
Alex: Explanation: No, you cannot. You must never rank different drugs by zone size. You have to compare each drug's zone to its own published cutoffs. Skill 7!
Sam: Scenario 3: Your friend wants to ask their doctor for antibiotics because they have a cold virus. What principle are they ignoring?
Think first, then show the answer
Alex: Explanation: Antibiotic stewardship. Antibiotics do not kill viruses, and unnecessary use selects for resistant bacteria.
Sam: Keep studying! Practice the 6 Rs: Record, Reduce, Review, Reflect, Recite, Revise.
MI Unit 1 - Audio 6: Debate on Antibiotic Stewardship
Alex: Welcome to Audio 6. Today we're doing something a little different: a debate on antibiotic stewardship. I'll take the side of aggressive treatment, and my co-host will take the side of strict conservation.
Sam: That's right. And to be clear, we are only using the facts from our Unit 1 sources. Let's get into it. You argue we should use antibiotics aggressively.
Alex: Yes. My position is that when a patient has an infection, we must act fast to stop it. If we use a broad approach or a strong dose early, we knock out the bacteria before the patient gets sicker.
Sam: But my position is that every time we use an antibiotic, we select for resistance. If we overprescribe, or use them when we aren't sure it's a bacteria, we leave behind the strong, resistant bugs. Those survive, multiply, and spread.
Alex: But if we are too strict, we risk a false negative scenario, missing a sick person who truly needs the medicine, which harms them directly.
Sam: True, but if we prescribe too freely, we get the equivalent of a false positive, treating people who don't have a bacterial infection, like giving antibiotics for a cold virus. This harms the public because it speeds up the creation of superbugs. That's why we must test first. We can use disk diffusion tests and find the MIC, the minimum inhibitory concentration, to pick the right drug at the lowest effective dose.
Alex: I agree that testing is important. If a drug's zone falls below that drug's own susceptible cutoff, the organism is not susceptible to it and we shouldn't use it. A small zone by itself does not decide; some drugs have small susceptible cutoffs, so we read the chart first.
Sam: Exactly. And we also agree that patients must finish a prescribed course only as their clinician directs.
Alex: So let's summarize the science we both agree on. First, antibiotics select for resistance. Every use has a cost.
Sam: Second, we must use tests like disk diffusion to measure susceptibility and find the MIC before throwing strong drugs at an infection.
Alex: Third, antibiotics don't work on viruses, so demanding them for a cold violates good stewardship.
Sam: And fourth, using them wisely protects everyone. It's a balance between treating the patient today and preserving the drugs for tomorrow.
Alex: Well debated! Keep these facts in mind for the test. Close out your studying with the 6 Rs: Record, Reduce, Review, Reflect, Recite, Revise.
MI Unit 1 - Audio 7: Night-Before Review of the 7 Skills
Alex: Welcome to Audio 7, the night-before review! The Unit 1 Take-Home Exam is this week.
Sam: It’s a take-home exam due Monday at the start of class. Section A is multiple choice with open notes, and you need to do the math by hand. Tonight, we’re going to quickly recap the 7 tested skills from the skills list and give you one memory hook for each.
Alex: Skill 1: Read an ELISA plate and trust it only when controls behave.
Sam: The Memory Hook: "Controls before patients." If the positive isn't colored or the negative isn't clear, stop. You can't trust the plate.
Alex: Skill 2: Calculate sensitivity and see what moving the cutoff does.
Sam: The Memory Hook: "The Seesaw." Sensitivity (catching the sick) and specificity (clearing the healthy) move in opposite directions. Lower the cutoff, sensitivity goes up, specificity goes down.
Alex: Skill 3: Do serial dilution math.
Sam: The Memory Hook: "Divide by 10, not 9." In a 1:10 dilution, the sample is the final volume divided by 10. The diluent is the rest.
Alex: Skill 4: Pair DNA bases and read BLAST results honestly.
Sam: The Memory Hook: "Similarity is not cause." A pairs with T, C pairs with G, no U in DNA. And judge a BLAST hit on coverage, identity, and E-value together.
Alex: Skill 5: Keep outbreak case files and contact maps honest.
Sam: The Memory Hook: "Unknown is not negative." If a lab result isn't there, you don't know it. And contact lines show connection, not direction.
Alex: Skill 6: Write notebook entries someone else can repeat, and remove gloves safely.
Sam: The Memory Hook: "Repeatable means exact." Use exact units and steps. And peel gloves inside-out safely.
Alex: Skill 7: Read antibiotic disk plates.
Sam: The Memory Hook: "Never rank different drugs." A smaller zone means a higher MIC for the same drug, but you can't compare zone sizes across different drugs. Read each drug's own chart.
Sam: Get a good night's sleep. Write your exam in pen. You've got this!
Alex: And one last time: Record, Reduce, Review, Reflect, Recite, Revise. Good luck on the exam!
Each walkthrough narrates one test from the first step to the signal you read. The gray line describes what you would see on screen.
Video 1 Script: ELISA Step by Step, with Controls
Title Card - "How to Read an ELISA: Step by Step"
Welcome to Medical Interventions. Today we are looking at the ELISA, which stands for enzyme-linked immunosorbent assay. We use it to look for a specific antibody in a patient's sample, like the antibodies your body makes to fight an infection. Let's see how it works step by step.
Animation of a clear plastic well being coated with spiky green shapes representing antigens. Then, a blue fluid fills the well and coats the remaining bare plastic.
We start with a plastic well. First, we coat the well with the germ's antigen, the target we are looking for. Then, we add a blocking solution to cover any bare plastic so nothing else sticks by accident.
A patient sample (yellow fluid) is pipetted into the well. Y-shaped antibodies (human antibodies) bind to the green antigens.
Next, the patient sample goes in. If the patient has antibodies against the infection, those Y-shaped antibodies will stick to the antigens.
A wash step, fluid flushes out the well, washing away unbound particles. The bound antibodies remain.
Now comes a critical step: the wash. Washing removes anything that did not bind.
A second type of antibody (red, with a small star attached) is added. It binds to the human antibodies.
Next, we add an enzyme-linked second antibody. This special antibody is designed to bind to human antibodies. Think of the enzyme as a tiny color-making machine waiting to be turned on.
Another wash step flushes the well. A warning symbol flashes briefly.
We wash the well again. This wash is vital! If you skip this wash, leftover enzyme-linked antibodies will float around and eventually color the well even if the patient didn't have the target antibody.
A colorless substrate is added. Where the chain is complete, the enzyme turns the substrate blue and the liquid in the well turns a bright blue color.
Finally, we add a colorless substrate. If the enzyme is present, it turns the substrate into a colored product. The signal is color: in this format, more bound antibody means more color. Real labs read this with a plate reader and compare it to a cutoff value.
Split screen. Left side: A "Positive Control" well turning blue. Right side: A "Negative Control" well staying clear.
But a test result is only as trustworthy as its controls. Before looking at any patient sample, we check the controls. The positive control, which we know has the antibody, must show color. The negative control, which we know lacks the antibody, must stay clear.
The Positive Control well stays clear. A big red "X" appears over the plate. Then, the Negative Control well turns blue. Another red "X" appears.
What if the positive control stays clear? That means a reagent or step failed, and you must repeat the test. What if the negative control turns colored? That’s background color, meaning something went wrong, like a missed wash. Again, you must repeat the test.
Final summary screen with bullet points: ELISA, Washes remove unbound targets, Controls must behave.
Only when both controls behave correctly can you compare your patient samples to the cutoff. Remember: wash carefully, and always check your controls!
Video 2 Script: Sensitivity, Specificity and Moving the Cutoff
Title Card - "Trading Errors: Sensitivity and Specificity"
Welcome back. Today we are talking about a big idea in Medical Interventions: every test trades two kinds of errors. No medical test is perfect, and we measure a test's accuracy using sensitivity and specificity. Let's break down what these mean.
A 2x2 table appears on screen. Rows are labeled "Test Positive" and "Test Negative". Columns are labeled "Actually Sick" and "Actually Healthy".
We build these concepts from a two-by-two table comparing the test result against the true status of the patient. Let's look at the four possible outcomes.
The top left cell highlights: "True Positive".
A "True Positive" is when a sick person correctly tests positive.
The bottom right cell highlights: "True Negative".
A "True Negative" is when a healthy person correctly tests negative.
The top right cell highlights in orange: "False Positive".
But what about the errors? A "False Positive" labels a healthy person as sick.
The bottom left cell highlights in red: "False Negative".
And a "False Negative" misses a sick person entirely, telling them they are healthy.
The table fades out. The word "Sensitivity" appears on screen with a math equation: True Positives / (Everyone who has the infection).
Sensitivity is the test's ability to catch the sick. We calculate it by taking the true positives and dividing them by everyone who actually has the infection. High sensitivity means very few false negatives.
The word "Specificity" appears on screen with an equation: True Negatives / (Everyone who does not have the infection).
Specificity is the test's ability to clear the healthy. We calculate it by taking the true negatives and dividing them by everyone who does not have the infection. High specificity means very few false positives.
A graph showing two overlapping bell curves. The left curve (blue) is "Healthy" and the right curve (red) is "Sick". A vertical dashed line labeled "Cutoff" separates positive from negative results.
Here is where the trade-off happens. Labs use a numerical "cutoff" to decide what counts as positive.
The cutoff line moves to the left (lowering the cutoff).
If we lower the cutoff, it is easier to test positive. Sensitivity goes up because we catch more sick people. But specificity goes down, meaning we get more false positives. We label more healthy people as sick.
The cutoff line moves to the right (raising the cutoff).
If we raise the cutoff, it is harder to test positive. Specificity goes up because we correctly clear more healthy people. But sensitivity goes down, meaning we get more false negatives, missing people who are actually sick.
Final summary screen with bullet points: Sensitivity catches the sick, Specificity clears the healthy, Moving cutoff trades errors.
You cannot usually improve both at the same time. Remember: lowering the cutoff increases sensitivity, while raising it increases specificity. Every test trades these two kinds of error.
Video 3 Script: Disk Diffusion and MIC
Title Card - "Testing Antibiotics: Disk Diffusion and MIC"
Welcome. Today we are looking at how to test if an antibiotic will work against a specific bacteria. This is called antibiotic susceptibility testing, or the disk diffusion method.
A petri dish (agar plate) is shown. A spreader spreads a liquid culture evenly over the entire surface.
We start with an agar plate, which is a shallow dish filled with nutrient gel. We spread a standardized amount of bacteria evenly over the whole surface to make what we call a "lawn."
Small white paper disks are placed onto the agar surface. The disks are labeled with letters like 'AMP', 'TET', etc.
Next, we place paper disks on top of the lawn. These disks are soaked in different antibiotics.
A clock spins rapidly to indicate time passing (incubation). As time passes, the bacteria grow, turning the plate cloudy. However, clear circles appear around some of the disks.
The plate incubates for about a day. As it sits, the antibiotic diffuses, or spreads out, from each disk into the gel. The bacteria grow everywhere they can.
The camera zooms in on one disk with a clear circle around it. A ruler appears and measures the diameter of the circle in millimeters.
The signal we are looking for is a clear ring around a disk where the bacteria could not grow. We call this the "zone of inhibition." To read the result, we measure the diameter of this clear zone in millimeters and compare it to that specific drug's published cutoffs to see if the bacteria are susceptible, intermediate, or resistant.
The screen splits. On the left, Drug A shows a small zone. On the right, the same Drug A shows a large zone.
When comparing the same drug on two different strains of bacteria, a smaller zone means the bacteria are less susceptible. It points to a higher MIC.
Text appears: "MIC: Minimum Inhibitory Concentration - the lowest concentration of a drug that stops visible growth."
MIC stands for Minimum Inhibitory Concentration. It is the lowest concentration of a drug needed to stop visible growth. A smaller zone of inhibition means it takes a higher concentration of the drug to stop the bacteria.
Two different drugs are shown on the plate. Drug A has a large zone, Drug B has a medium zone. A big red "X" appears over a thought bubble that says "Drug A is better than Drug B".
A critical rule: never rank different drugs just by looking at their zone sizes! Different drugs spread through the gel at different speeds. You must read each drug against its own standard chart.
Final summary screen: Disk Diffusion makes a lawn, Zone of Inhibition = clear ring, Never rank different drugs by zone size, Stewardship matters.
Remember, antibiotics select for resistance. Using them wisely, a practice called stewardship, protects everyone. Measure carefully, and use the right drug for the job.
Video 4 Script: Serial Dilution and BLAST
Title Card - "Dilutions and DNA: Math and BLAST"
Hello. Today we are combining two essential lab skills: serial dilutions and DNA sequencing analysis. First, let's talk about dilution math.
A test tube with a dark blue liquid labeled "Sample". Next to it, a test tube with a light blue liquid labeled "1:10 Dilution".
A serial dilution is a step-by-step way to make a sample weaker by a known factor. This helps us find out how much of something is present, or brings a strong sample into a readable range.
An animation shows 1 mL of sample being pipetted into 9 mL of clear diluent in a new tube. The new tube is labeled "1:10".
Let's look at a 1 to 10 dilution. A 1 to 10 dilution means 1 part sample in 10 parts total. This means you mix 1 part of the sample with 9 parts of diluent.
A math equation appears: Sample Volume = Final Volume / 10. Diluent Volume = Final Volume - Sample Volume.
To find your volumes, divide your final total volume by 10 to get your sample volume. The rest is diluent.
A warning sign flashes. Text appears: "TRAP 1: Dividing by 9" and "TRAP 2: Adding 10 parts diluent".
Watch out for common traps! Do not divide your final volume by 9. And do not add 1 part sample to 10 parts diluent, that makes 11 parts total, a 1 to 11 dilution. Each 1 to 10 step divides the concentration by exactly 10.
The scene shifts from tubes to a DNA double helix. The letters A, T, C, and G appear.
Now, let's shift to DNA. When we sequence DNA, we read the order of bases: A, T, C, and G. Remember the base pairing rules: A always pairs with T, and C always pairs with G. And importantly, there is no U in DNA!
A computer screen shows a DNA sequence being entered into a search bar labeled "BLAST".
We take this sequence and put it into a tool called BLAST, which compares our read with a huge database of known sequences to find the closest match.
A BLAST results table highlights three columns: "Query Coverage", "Percent Identity", and "E-value".
When reading a BLAST hit, you must look at three numbers together. Query coverage tells you how much of your sequence matched. Percent identity tells you how exactly the letters matched. And E-value tells you how many matches this good you would expect by chance; you want this number to be as close to zero as possible.
A result shows 100% Identity, but only 5% Query Coverage. A red warning icon appears.
A 100 percent identity hit might look great, but if it's over a tiny piece of the sequence, like a low query coverage, it is a weak hit.
Final summary screen: 1:10 = 1 part sample + 9 parts diluent, A pairs with T (No U in DNA), Read BLAST with Coverage, Identity, and E-value, Strong match is not proof of cause.
And always remember a big idea of this unit: a strong match shows similarity, not cause. Just because a sequence matches an organism doesn't prove that organism caused the patient's illness. It's a clue, not a final diagnosis.
Why this matters beyond the test
- As a patient: ask what a test result means, whether it was confirmed, and whether you really need an antibiotic; finish a prescribed course only as your clinician directs.
- Life choices: never pressure a doctor for antibiotics for a cold or flu (viruses); protect health privacy; wash hands.
- Careers: medical laboratory scientist, microbiologist, epidemiologist, infection preventionist, pharmacist, public health nurse, physician, bioinformatics scientist.
Questions? Ask me in class or message me in Schoology. Go Hornets! Mr. Mendoza