Question

If we have a plate that has 2500 colonies on it, can we use this plate to calculate CFU's

Answer 1

No, the solution with bacteria in it needs to be diluted in order to be in the range of 30 – 300.

A colony-forming unit –CFU is used to estimate the number of viable (ability to divide by binary fission) microbes in a sample.  

A sample of microbes is diluted and spread on a plate. Then, the number of colonies is counted, assuming that each colony have grown from a single colony-forming unit, or CFU.

Plates with more than 300 colonies are difficult to count while those with less than 30 colonies give statistically unreliable numbers of colonies to count.