Microbiology & the Working Lab
The bench skills the WebXam leans on hardest, that a design capstone does not get to in class, for extra credit.
Our BI year is a design capstone, so we spend class time on innovation and projects. But the 072125 exam is mostly microbiology and culturing: Gram staining, plating, and testing. These four missions cover exactly those bench skills, so you walk into the exam having seen them. Everything is optional, self-paced, and from home. Each mission shows a worked example first.
This opens after the prototype unit. The 072125 exam is mostly microbiology and culturing, but our design capstone does not get to those bench skills in class, so this project covers them. It is due by the WebXam review window in May, which leaves about three months of runway.
Plan about 30 to 60 minutes each. That is about 3 to 4 hours total, and you have months to do it at your own pace. There is no daily deadline; chip away at it.
You may do this solo or in a small group. If you choose a group, submit ONE shared product per mission, plus a one-line note from each person saying what they did. The science bar is the same either way; the group rubric just adds a teamwork score. Pick the path that fits your schedule.
Mission 1: Gram Staining & Bacterial Morphology
The Gram stain sorts bacteria into two big groups by their cell wall, the first clue to what a microbe is and how to treat it.
Goal: Explain the Gram stain steps and classify bacteria as or by color and by shape.
- cells have a thick wall and stain purple (they keep the ).
- cells have a thin wall plus an outer membrane and stain pink (the counterstain).
- The steps in order: , then iodine (a mordant that traps the dye), then an alcohol wash (decolorize), then .
- Shape matters too: cocci are spheres, bacilli are rods, spirilla are spirals.
- Watch or read one explainer below.
- Write the four Gram stain steps in order and say what each one does.
- Classify three described samples by color and shape (for example, 'purple clusters of spheres').
- You can list the Gram stain steps in order.
- You can tell from by color and explain why the colors differ.
- You can name a cell's shape from a description.
Use this model to check your own work. Match the structure and detail, but make the wording and any data your own.
Gram stain, in order:
- Crystal violet: stains all cells purple.
- Iodine: a mordant that locks the dye into the thick walls.
- Alcohol wash: decolorizes thin-walled cells (they lose the purple).
- Safranin: counterstains the now-colorless thin-walled cells pink.
Classifying Sample 1: purple clusters of spheres = Gram-positive cocci. Because it kept the crystal violet, it has a thick peptidoglycan wall, and the grape-like clusters suggest a Staphylococcus.
| Result | Color | Wall | Example shape |
|---|---|---|---|
| Gram-positive | Purple | Thick peptidoglycan | Cocci (spheres) |
| Gram-negative | Pink | Thin wall + outer membrane | Bacilli (rods) |
This model shows the level of evidence and organization needed to complete: Completes the identification task: the Gram stain procedure and a classification of unknown samples by color and shape.
- State the question and method.
- Present the observations and data with units.
- Explain the result, limitations, and next investigation.
Keep the structure. Replace the question, facts, measurements, and evidence. Then recheck units, vocabulary, and whether the conclusion goes beyond the evidence.
Also due today: Submit your steps and the three classifications.
One exam-style question on this system. Try it before you reveal the answer, then read why each choice is right or wrong.
Tap an answer to see the full explanation. Nothing is recorded or graded.
It builds this reusable test skill: Reading a Gram stain result.
- Name the concept or data pattern being tested.
- Cross out choices that violate that rule or the evidence.
- Justify the best remaining choice before checking the answer.
Go to Schoology to turn this in. Submit one PDF. Put your first and last name in the document header. Name the file: FirstName LastName - Assignment Title - YYYY-MM-DD.pdf. If you cannot get in, see Mr. Mendoza. Do not skip the work.
Optional extra credit. This is the single most tested microbiology skill on the exam.
Mission 2: Culturing, Streak Plates & Colony Counts
Streak plating spreads cells out so each grows into one pure , and counting colonies lets you estimate how many cells were in a sample.
Goal: Explain why we streak for and calculate the concentration of bacteria (CFU per mL) from a countable plate.
- A drags cells across the agar so they get more and more spread out, until single cells grow into isolated colonies.
- Each grew from one founding cell, so a colony is a pure clone (a , CFU).
- Only plates with about 30 to 300 colonies are reliable to count: fewer is statistically shaky, more is too crowded to count.
- CFU per mL = colonies counted / (volume plated in mL x ).
- Watch or read one explainer below.
- Describe the streak-plate pattern and why later streaks have fewer cells.
- Calculate CFU per mL for the worked numbers, then explain which plate you would trust.
- You can explain how a isolates single colonies.
- You can pick the countable plate (30 to 300 colonies).
- You can calculate CFU per mL from a dilution plate.
Use this model to check your own work. Match the structure and detail, but make the wording and any data your own.
I plated 0.1 mL from a 1 in 10,000 dilution (dilution factor 0.0001) and counted 150 colonies.
CFU per mL = 150 / (0.1 mL x 0.0001) = 150 / 0.00001 = 1.5 x 10^7 CFU per mL.
I trust this plate because 150 is inside the countable range (30 to 300). The less-diluted plate was a solid lawn (too many to count) and the most-diluted plate had only 4 colonies (too few to be reliable).
| Plate (dilution) | Colonies | Countable? |
|---|---|---|
| 1 in 100 | lawn (uncountable) | No, too many |
| 1 in 10,000 | 150 | Yes (30 to 300) |
| 1 in 1,000,000 | 4 | No, too few |
This model shows the level of evidence and organization needed to complete: Completes the culturing task: a CFU per mL calculation from a countable plate with the choice of plate justified.
- Name the variables and include units.
- Enter observations without changing the raw values.
- Check labels, calculations, and patterns before interpreting the data.
Keep the structure. Replace the question, facts, measurements, and evidence. Then recheck units, vocabulary, and whether the conclusion goes beyond the evidence.
Also due today: Submit your calculation and your reason for choosing that plate.
One exam-style question on this system. Try it before you reveal the answer, then read why each choice is right or wrong.
Tap an answer to see the full explanation. Nothing is recorded or graded.
It builds this reusable test skill: Choosing a countable plate for colony counting.
- Name the concept or data pattern being tested.
- Cross out choices that violate that rule or the evidence.
- Justify the best remaining choice before checking the answer.
Go to Schoology to turn this in. Submit one PDF. Put your first and last name in the document header. Name the file: FirstName LastName - Assignment Title - YYYY-MM-DD.pdf. If you cannot get in, see Mr. Mendoza. Do not skip the work.
Optional extra credit. Culturing and counting show up across the exam.
Mission 3: Antibiotic Susceptibility (Kirby-Bauer)
The disk test shows which antibiotics stop a given bacterium by measuring the clear zone where it could not grow.
Goal: Interpret zones of to decide whether a bacterium is susceptible or resistant, and recommend the best .
- diffuses out of a paper disk into the agar; where it stops growth, you see a clear ring called the .
- A larger zone means the bacterium is more susceptible to that .
- You measure the zone diameter in millimeters and compare it to a standard chart to label it Susceptible, Intermediate, or Resistant.
- No zone (growth right up to the disk) means the bacterium is resistant to that drug.
- Watch or read one explainer below.
- Read the sample zone measurements and label each S, I, or R using the idea that bigger zone = more susceptible.
- Recommend the you would use and say why.
- You can explain what a is.
- You can rank antibiotics by susceptibility from their zone sizes.
- You can recommend a treatment from the data.
Use this model to check your own work. Match the structure and detail, but make the wording and any data your own.
I measured the clear zone around each disk and compared to the standard chart.
Recommendation: use Antibiotic C. It produced the largest zone of inhibition (22 mm = Susceptible), which means it stopped this bacterium best. Antibiotic A had no zone, so the bacterium is resistant to it, and B was only intermediate.
| Antibiotic | Zone (mm) | Interpretation |
|---|---|---|
| A | 0 | Resistant |
| B | 14 | Intermediate |
| C | 22 | Susceptible (best choice) |
This model shows the level of evidence and organization needed to complete: Completes the susceptibility task: zones interpreted as S/I/R and a defended antibiotic choice.
- Name the variables and include units.
- Enter observations without changing the raw values.
- Check labels, calculations, and patterns before interpreting the data.
Keep the structure. Replace the question, facts, measurements, and evidence. Then recheck units, vocabulary, and whether the conclusion goes beyond the evidence.
Also due today: Submit your S/I/R labels and your recommendation with a reason.
One exam-style question on this system. Try it before you reveal the answer, then read why each choice is right or wrong.
Tap an answer to see the full explanation. Nothing is recorded or graded.
It builds this reusable test skill: Interpreting zones of inhibition in a disk diffusion test.
- Name the concept or data pattern being tested.
- Cross out choices that violate that rule or the evidence.
- Justify the best remaining choice before checking the answer.
Go to Schoology to turn this in. Submit one PDF. Put your first and last name in the document header. Name the file: FirstName LastName - Assignment Title - YYYY-MM-DD.pdf. If you cannot get in, see Mr. Mendoza. Do not skip the work.
Optional extra credit. This connects microbiology to real treatment decisions.
Mission 4: Aseptic Technique & Lab SOP
keeps cultures pure and people safe, and standard procedures make results trustworthy and repeatable.
Goal: Explain and write a standard operating procedure for safely transferring and disposing of microbes.
- prevents unwanted microbes from contaminating your (and prevents your culture from contaminating you).
- Core moves: flame the before and after use, work near the flame, do not set caps down on the bench, and the bench before and after.
- Contaminated cultures give invalid results, so asepsis protects the science too.
- Used cultures are decontaminated (for example, autoclaved) before disposal; never put live cultures in the trash.
- Watch or read one explainer below.
- Write a numbered SOP for transferring bacteria from one tube to another using .
- Add a short disposal SOP and explain one thing that would contaminate the .
- You can explain why matters for and for valid results.
- You can write the steps of an transfer.
- You can describe safe decontamination and disposal.
Use this model to check your own work. Match the structure and detail, but make the wording and any data your own.
Aseptic transfer SOP:
- Disinfect the bench and wash hands.
- Flame the inoculating loop until red, then let it cool a few seconds.
- Open the source tube near the flame; do not set the cap on the bench.
- Touch the cooled loop to one colony, then streak it into the new tube, working near the flame.
- Flame the loop again before setting it down, and recap both tubes.
Disposal: place used cultures in the biohazard bag to be autoclaved; never pour live cultures down the sink or into the trash. Contamination risk: setting the cap down on the bench can pick up stray microbes and ruin the culture.
This model shows the level of evidence and organization needed to complete: Completes the SOP task: a numbered aseptic transfer procedure and a safe disposal note.
- State the question and method.
- Present the observations and data with units.
- Explain the result, limitations, and next investigation.
Keep the structure. Replace the question, facts, measurements, and evidence. Then recheck units, vocabulary, and whether the conclusion goes beyond the evidence.
Also due today: Submit your transfer SOP and disposal note.
One exam-style question on this system. Try it before you reveal the answer, then read why each choice is right or wrong.
Tap an answer to see the full explanation. Nothing is recorded or graded.
It builds this reusable test skill: Applying aseptic technique.
- Name the concept or data pattern being tested.
- Cross out choices that violate that rule or the evidence.
- Justify the best remaining choice before checking the answer.
Go to Schoology to turn this in. Submit one PDF. Put your first and last name in the document header. Name the file: FirstName LastName - Assignment Title - YYYY-MM-DD.pdf. If you cannot get in, see Mr. Mendoza. Do not skip the work.
Optional extra credit. Good lab procedure is its own exam domain and underpins every other mission.
Pick the rubric that matches how you worked. The science bar is the same for both; the group rubric just adds a teamwork score. Each criterion is worth up to 4 points.
Each mission is scored out of 16 (four criteria, 4 points each). This is extra credit, so the goal is to show what you learned.
| Criterion | Exemplary (4) | Proficient (3) | Developing (2) | Beginning (1) |
|---|---|---|---|---|
| Science accuracy | All of the science is correct and precise, with no errors. | The science is correct, with at most one small slip. | Several ideas are partly correct, but there are some clear errors. | Major errors, or key ideas are missing. |
| Completeness | Every required part is present and thorough (diagram or table AND the write-up). | All parts are present; one is a little thin. | A required part is missing or very thin. | Most required parts are missing. |
| Structure (uses the example) | Clearly follows the worked example's structure and puts it in your own words. | Follows the structure of the worked example. | Loosely follows the structure. | Does not follow the structure. |
| Clarity and vocabulary | Clear and well organized; key terms are used correctly throughout. | Clear; key terms are mostly used correctly. | Some parts are unclear, or some vocabulary is misused. | Hard to follow; key vocabulary is missing or incorrect. |
Each mission is scored out of 20 (five criteria, 4 points each). Submit one shared product plus a one-line contribution note from each member.
| Criterion | Exemplary (4) | Proficient (3) | Developing (2) | Beginning (1) |
|---|---|---|---|---|
| Science accuracy | All of the science is correct and precise, with no errors. | The science is correct, with at most one small slip. | Several ideas are partly correct, but there are some clear errors. | Major errors, or key ideas are missing. |
| Completeness | Every required part is present and thorough (diagram or table AND the write-up). | All parts are present; one is a little thin. | A required part is missing or very thin. | Most required parts are missing. |
| Structure (uses the example) | Clearly follows the worked example's structure and puts it in your own words. | Follows the structure of the worked example. | Loosely follows the structure. | Does not follow the structure. |
| Clarity and vocabulary | Clear and well organized; key terms are used correctly throughout. | Clear; key terms are mostly used correctly. | Some parts are unclear, or some vocabulary is misused. | Hard to follow; key vocabulary is missing or incorrect. |
| Collaboration and shared contribution | Clear evidence every member contributed; roles are named and the product reads as one cohesive piece. | All members contributed and the product is cohesive. | Contribution was uneven, or the product feels stitched together from separate parts. | One person did most of the work, or the contribution notes are missing. |