What is the acceptable range of CFU per plate to consider a plate viable?

What is the acceptable range of CFU per plate to consider a plate viable?

• A. 10-50 CFUs
• B. 30-300 CFUs
• C. 50-500 CFUs
• D. 100-1000 CFUs

Answer: (B)

Counting viable colonies works best when the plate shows a countable number of colonies, not too few and not too many. The range of 30–300 CFUs per plate is used because it provides enough colonies to average out random counting errors while avoiding crowding that makes individual colonies hard to distinguish. If there are fewer than about 30 colonies, small counting errors become large relative to the total, skewing the result. If there are more than about 300 colonies, colonies can merge or overlap, making accurate counts difficult and leading to underestimation when calculating the original concentration. So 30–300 CFUs per plate balances reliability and practicality for estimating CFU in the original sample.

Counting viable colonies works best when the plate shows a countable number of colonies, not too few and not too many. The range of 30–300 CFUs per plate is used because it provides enough colonies to average out random counting errors while avoiding crowding that makes individual colonies hard to distinguish. If there are fewer than about 30 colonies, small counting errors become large relative to the total, skewing the result. If there are more than about 300 colonies, colonies can merge or overlap, making accurate counts difficult and leading to underestimation when calculating the original concentration. So 30–300 CFUs per plate balances reliability and practicality for estimating CFU in the original sample.