How to calculate CFU using logarithms

Question

Suppose that you want to determine the bacteria/ml (CFU) for a food sample. You decide to make dilutions and then plate 250µL of the dilutions listed below. After 24 hrs of incubation you obtain the plate counts listed below. Based on the information provided calculate the CFU for the food sample.

Dilution Plate count
10-4	480
10-5	350
10-6	150
10-7	70

Answer 1

concentration=(CFU)/(dilution factor*volume plated) You want to use the CFU from a plate with between 30 and 300 CFU. I would use the plate from the 10-6 dilution, as the larger number of colonies will make a more accurate calculation.