John Hay
John Hay Biomedical
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Genetics of Disease (Medical Interventions)
Unit 1: Unit 1.2 Culturing & ResistanceMI 1.2Culturing

Culture and Count Colonies

Use culture and antibiotic evidence to culture and count colonies.

Builds on (2 levels back)inferred · high confidence
  • Aseptic culture technique: Culture results need uncontaminated samples.
  • Antibiotic selection pressure: Drug exposure can favor resistant bacteria.

Prerequisites are inferred: pending teacher review.

Re-learn the skill with worked practice and clear examples.

Compute CFU/mL from a real plate count using colonies / volume x dilution factor.

Step 1: Read the plate data
You are given colonies counted, the volume plated, and the dilution factor.
ItemValue
Colonies counted on plate50
Volume plated0.1 mL
Dilution factor1000 (1:1000 dilution)
CFU/mL formulacolonies / volume plated x dilution factor
Plate data: 50 colonies counted, 0.1 mL plated, from a 1:1000 dilution, with the CFU/mL formula.
Step 2: Plug in the formula
CFU/mL = colonies / volume plated x dilution factor. Divide by the volume first, then multiply by the dilution factor.
Step 3: Check the size
A 1:1000 dilution means the original sample is much more concentrated than the plate, so expect a large number.
Practice

Using the plate data (50 colonies, 0.1 mL plated, 1:1000 dilution), what is the CFU/mL of the original sample? Use CFU/mL = colonies / volume x dilution factor.

Reviewed
ItemValue
Colonies counted on plate50
Volume plated0.1 mL
Dilution factor1000 (1:1000 dilution)
CFU/mL formulacolonies / volume plated x dilution factor
Plate data: 50 colonies counted, 0.1 mL plated, from a 1:1000 dilution, with the CFU/mL formula.
  1. A.5 x 10^5 CFU/mL
  2. B.5 x 10^3 CFU/mL
  3. C.5 x 10^2 CFU/mL
  4. D.50 CFU/mL
Show the worked solution ▾

Answer: A. 5 x 10^5 CFU/mL

  1. Step 1: Divide by volume: 50 / 0.1 = 500.
  2. Step 2: Multiply by dilution factor: 500 x 1000 = 500,000 = 5 x 10^5.

Why it's right: 50 / 0.1 = 500, and 500 x 1000 = 500,000, which is 5 x 10^5 CFU/mL.

Why the others miss:
  • B: This skips the divide-by-0.1 step (it uses 50 x 1000 / ... incorrectly).
  • C: This forgets the dilution factor.
  • D: This is just the raw colony count with no calculation.

Aligned to Culturing · reading level ~grade 9

Where you'd see this
  • In Unit 1.2 Culturing & Resistance, this skill turns class evidence into a result another person can check.
Careers that use this
Microbiologist: Uses culture and count colonies to make a decision from evidence without overclaiming.
Video library
Watch: Culture and Count Colonies
How to Calculate CFU per ml of Bacterial Sample? in 3 Steps || cfu/ml in Microbiology
biologyexams4u · ~7 min
Guided notes

Fill these in as you work through the lesson.

Big idea: You find how many bacteria were in a sample by counting colonies and scaling up with the volume plated and the dilution factor.
Key terms: write the meaning
  • Colony (a visible spot that grew from ____ living cell):  
  • CFU (colony-forming ____, one per colony):  
  • Dilution factor (how many ____ the sample was diluted (e.g., 1000 for 1:1000)):  
  • Volume plated (the ____ of liquid spread on the plate, in mL):  
The rule

CFU/mL = colonies counted   volume plated   the dilution factor.

Check yourself
  1. How many CFU does one colony represent? 
  2. What two numbers do you scale the colony count by? 
  3. Do you divide by the volume before or after multiplying by the dilution factor? 
Work one example

A plate has 25 colonies from 0.1 mL of a 1:1000 dilution. Show each step (divide by 0.1, then multiply by 1000) to find the CFU/mL.