Question

A culture of E. coli cells (1 ml) was diluted \(10^6\) fold and 100 $\mu$l of this was used for plating. After 24 hours incubation, the number of colony forming units (CFU) was found to be 180. The CFU count of the original culture is

• A. \(1.8 \times 10^9\) CFU/ml
• B. \(1.8 \times 10^6\) CFU/ml
• C. \(1.8 \times 10^7\) CFU/ml
• D. \(1.8 \times 10^8\) CFU/ml

Hint:

CFU/ml = colonies ÷ plated volume × dilution factor Always convert $\mu$l → ml correctly!

Answer 1

The Correct Option is A

Concept: \[ \text{CFU/ml} = \frac{\text{Number of colonies}}{\text{Volume plated (ml)}} \times \text{Dilution factor} \]

Step 1: Convert volume.
\[ 100~\mu l = 0.1~ml \]

Step 2: Substitute values.

• Colonies = 180
• Volume plated = 0.1 ml
• Dilution factor = \(10^6\)

Step 3: Calculate CFU/ml.
\[ \text{CFU/ml} = \frac{180}{0.1} \times 10^6 \] \[ = 1800 \times 10^6 = 1.8 \times 10^9 \]

Step 4: Final conclusion.
Thus, CFU count of original culture is: \[ {1.8 \times 10^9~\text{CFU/ml}} \]