Question

Assume that one counted 67 plaques on a bacterial plate where 0.1 ml of a 105 dilution of phage was added to

bacterial culture. What is the initial concentration of the undiluted phage?
A) 6.7 × 105 pfu/ml
B) 0.1 × 105 pfu/ml
C) 6.7 × 105 pfu/ml
D) 67 pfu/ml
E) 6.7 × 107 pfu/ml (pfu = plaque-forming units)

Answer 1

To calculate the initial phage concentration from a count of 67 plaques in a 0.1 ml sample of a 10⁵ dilution of phage, multiply the plaque count by 10⁶. The result is 6.7 × 10⁷ pfu/mL.

Step-by-step explanation:

If 67 plaques were counted on a bacterial plate from a 0.1 ml sample of a 105 dilution of phage, we would need to find the initial phage concentration in the undiluted solution.

First, we consider the dilution factor which is 105, and since 0.1 mL of this diluted sample was used, we multiply the count of plaques by 10⁶ (10⁵ for the dilution and 10 to account for the 0.1 mL sample volume) to get the phage concentration per mL. Hence, the initial phage concentration would be 67 plaques × 10⁶, which is 6.7 × 10⁷ pfu/mL.