Question

Ten litres of the sample was filtered through a membrane filter and the filter was transferred to solid agar media which supports growth of coliform organisms. After incubation at 37°C for 48 hours, the agar plates showed an average of 64 colonies per plate. What was the average concentration of coliform organisms (in CFU/ml) in the original water sample?

• A. 64 CFU/ml
• B. 640 CFU/ml
• C. $6.4 \times 10^{-3}$ CFU/ml
• D. $64 \times 10^{-3}$ CFU/ml

Hint:

CFU/ml = colonies counted ÷ total volume filtered (in ml).

Answer 1

The Correct Option is C

Total water filtered = 10 L Convert to ml: \[ 10\ \text{L} = 10 \times 1000 = 10000\ \text{ml}. \] The filter collects all organisms from these 10000 ml, and produces 64 CFU on the agar plate. Thus the concentration is: \[ \text{CFU/ml} = \frac{64}{10000} = 0.0064 = 6.4 \times 10^{-3}. \] Therefore, the average concentration of coliform organisms in the sample is \[ 6.4 \times 10^{-3}\ \text{CFU/ml}. \] Final Answer: (C)