CFU stands for Colony Forming Units, a microbiology term used to quantify how many bacteria exist in a solution. Depending on the concentration of your sample, you need to perform multiple dilutions and plate the different samples onto petri dishes. If you have too many bacterial colonies, they are hard to count, and if there are too few, the sample may not be representative. It is generally a good idea to plate the original solution, then a 1/10 dilution (1 part solution, 9 parts saline), a 1/100 dilution and possibly a 1/1000 dilution.
Calculating CFU from Bacterial Dilution
Perform Preliminary Count
Perform a preliminary count of each dish once the bacteria incubates, which usually takes one or two days. Count only individual colonies, which should be distinct, isolated dots, not a whole blob of different colonies grown together. Choose the plate which has more than 30 of these colonies but less than 300.
Count Individual Colonies
Count the number of individual colonies. This is the CFU number of your dilution -- you will have to perform a simple calculation to determine the CFU of the original sample. For this example, use a hypothetical plate containing 46 colonies.
Determine Size of Dilution
Determine the size of the dilution you used. (Ideally, you labeled the petri dishes ahead of time.) For this example, mix 1 mL of bacterial culture with 99 mL of saline. This is a 1/100 dilution.
Multiply Degree of Dilution by Amount Plated
Multiply the degree of the dilution by the amount you actually plated. If you plated 0.1 mL of your 1/100 dilution onto the agar, you multiply 0.1 x 1/100, for a result of 1/1000 or 0.001.
Divide CFU of Dilution
Divide the CFU of the dilution (the number of colonies you counted) by the result from step 4. For this example, you work out 46 ÷ 1/1000, which is the same as 46 x 1,000. The result is 46,000 CFU in the original sample.