Question

Forty-eight plaques were counted in a plate when 1 ml of bacteriophage-infected bacteria from a 10-6 dilution of a 10-ml culture was plated. What was the original bacteriophage density?

Options:
a) 480 pfu/ml
b) 480,000 pfu/ml
c) 4.8 x 10^6 pfu/ml
d) 4.8 x 10^7 pfu/ml

Answer 1

The original bacteriophage density is calculated by multiplying the number of plaques (48) by the dilution factor (10^6) and then dividing by the volume plated (1 mL), resulting in 4.8 × 10^7 pfu/mL.

Step-by-step explanation:

To determine the original bacteriophage density, one must consider the dilution factor and the number of plaques formed. In the question, 48 plaques were counted on a plate that represents a 10-6 dilution of a 10-ml culture. To calculate the density of bacteriophage in the original sample, one multiplies the number of plaques by the dilution factor and then by the inverse of the volume plated. The calculation is as follows:

1. Number of plaques counted: 48
2. Dilution factor: 106 (because the sample was from a 10-6 dilution)
3. Volume plated: 1 mL
4. Original bacteriophage density = plaques × dilution factor / volume plated
5. Original bacteriophage density = 48 × 106 / 1 = 4.8 × 107 pfu/mL

The correct answer is (d) 4.8 × 107 pfu/mL.