Question

A landfill sample was analyzed by dilution and plating techniques for viable bacterial count. When one gram of the landfill sample was diluted (1 times 10^4) (w/v) it yielded 400 CFU. The viable bacterial count (in million, rounded off to the nearest integer) in one gram landfill sample is ____.

Hint:

When calculating microbial counts from diluted samples, always ensure the dilution factor is accurately used to revert the concentration to the original sample volume or weight for an accurate estimate of microbial load.

Answer 1

To find the original bacterial count in the landfill sample, we use the formula: \[ \text{Viable count per gram} = \frac{\text{Number of colonies} \times \text{Dilution factor}}{\text{Amount of sample plated (in grams)}} \] Step 1: Given values.

• Number of colonies (CFU) = 400
• Dilution factor = \( 10^4 \)
• Amount of sample plated = 1 gram (since it mentions 1 gram was diluted)

Step 2: Calculating the viable count. \[ \text{Viable count per gram} = \frac{400 \times 10^4}{1} = 4,000,000 \text{ CFU per gram} \] Step 3: Converting to millions. \[ \text{Viable count in million} = \frac{4,000,000}{1,000,000} = 4 \text{ million CFU per gram} \] Conclusion:

Explanation: The calculation is straightforward: multiply the number of colonies by the dilution factor to revert the dilution effect and get the count per original sample volume or weight.