Q82. A bacterial culture was diluted 1000 fold and 0.1 ml of this diluted sample was spread per plate on nutrient agar. In a triplicate run, the number of colonies formed is 121, 93 and 86. The number of colony forming units/ml in the original bacterial culture is
CFU/ml Calculation: 1000-Fold Dilution, 0.1 ml Plate (121, 93, 86 Colonies) | Correct Answer 106
A bacterial culture was diluted 1000-fold, and 0.1 ml of this dilution yielded colony counts of 121, 93, and 86 in triplicate plates.
The correct number of colony forming units per ml (CFU/ml) in the original culture is 106
Step-by-Step Calculation
Average the triplicate colony counts first: (121 + 93 + 86) / 3 = 300 / 3 = 100 colonies.
CFU per plate equals this average, as each colony arises from a viable cell under ideal conditions.
Account for plating volume and dilution: CFU/ml in diluted sample = average colonies / volume plated × dilution factor = 100 / 0.1 ml × 1000 = 106 CFU/ml.
Why 106 CFU/ml?
This value fits standard viable cell enumeration, where dilutions ensure 30–300 countable colonies (here, 86–121 is ideal).
Option Analysis
| Option | Calculation Error | Why Incorrect |
|---|---|---|
| 106 | (100 / 0.1) × 103 = 106 | Correct: Matches formula for original culture density. |
| 105 | Ignores plating volume (treats as 1 ml) | Underestimates by factor of 10. |
| 103 | Forgets dilution factor entirely | Gives diluted density only. |
| 102 | Divides by volume and dilution wrongly | Severe undercount. |
Key Applications in Microbiology
CFU/ml quantifies live bacteria for research, food safety, and infection studies, always using triplicates for precision.
