Plating 10 mL of a sample diluted by a factor of 103 produce

Plating 1.0 mL of a sample diluted by a factor of 10^-3 produced 43 colonies. What was the original cell density in the sample? Plating 0.1 mL of a sample diluted by a factor of 10^-3 produced 43 colonies. What was the original cell density in the sample?

Solution

original cell density = CFU/ original sample volume

1. 4.3 x 10^4 CFU/ml

2. 4.3x 10^5 CFU/ml