SME
Microbiology · Problem type
Viable plate count from a dilution series
Use this for “156 colonies grow from 0.1 mL of the 10⁻⁶ dilution. How many CFU/mL were in the culture?”
Diagram
Each tube is a 1:10 dilution of the one before (1 part into 9 parts of broth).
CFU/mL
=
colonies
dilution × volume
=
156
1 ×
10
-6
× 0.1
=
1.56 ×
10
9
CFU/mL
This is an example. Type your own values to start a new problem.
C (CFU/mL)
=
colonies
×
V (mL)
Formulas
C
=
colonies
dilution ×
V
1.56 ×
10
9
=
156
1 ×
10
-6
× 0.1
Assumptions
•
Each colony grew from one cell or clump, a colony-forming unit (CFU).
•
Only plates with 30 to 300 colonies are counted: with fewer, chance swamps the count; with more, colonies merge. Outside that range, count another plate.
•
Each tube is 1 mL into 9 mL of broth, a 1:10 dilution of the one before.
Step-by-step
WE KNOW
colonies = 156, dilution = 1 × 10⁻⁶, V = 0.1 mL
FIND
culture density (C)
Step 1 · Find culture density (C)
Use C = colonies ÷ (dilution × V)
Each colony was one cell in the volume plated of that dilution.
C
=
156
1 ×
10
-6
× 0.1
C
=
156
10
-7
C
=
1.56 ×
10
9
CFU/mL
Check
Put the numbers back in. Both sides should match.
1.56 ×
10
9
=
156
1 ×
10
-6
× 0.1
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
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