What is the acceptable range of colonies for accurate CFU counting on a plate?

What is the acceptable range of colonies for accurate CFU counting on a plate?

• A. 10-50
• B. 30-300
• C. 300-1000
• D. 1-5

Answer: (B)

When estimating CFU per mL, you want a plate with a countable number of colonies so the data are statistically reliable. The sweet spot is about 30 to 300 colonies on a plate. Why this range works is that it keeps counting accuracy high while avoiding colony merging. Counting follows Poisson-like variation, so the uncertainty goes down as you have more colonies; around 30 colonies gives a reasonable relative error, and around 300 keeps the error small without crowding. If you have too few colonies, the relative error becomes large because random variation dominates; if you have too many, colonies start to overlap or crowd, making individual counts impossible and underestimating the true number. With proper dilution and plating, you multiply the counted colonies by the reciprocal of the dilution factor and the plated volume to get CFU per mL, and using a count in this range yields a more accurate estimate.

When estimating CFU per mL, you want a plate with a countable number of colonies so the data are statistically reliable. The sweet spot is about 30 to 300 colonies on a plate. Why this range works is that it keeps counting accuracy high while avoiding colony merging. Counting follows Poisson-like variation, so the uncertainty goes down as you have more colonies; around 30 colonies gives a reasonable relative error, and around 300 keeps the error small without crowding. If you have too few colonies, the relative error becomes large because random variation dominates; if you have too many, colonies start to overlap or crowd, making individual counts impossible and underestimating the true number. With proper dilution and plating, you multiply the counted colonies by the reciprocal of the dilution factor and the plated volume to get CFU per mL, and using a count in this range yields a more accurate estimate.