How to calculate Colony Forming Units (CFU)

CFU stands for Colony-Forming Unit. In microbiology, CFU is a measurement used to estimate the number of viable microorganisms (bacteria or yeast) in a given sample. It represents the number of individual cells or cell clusters that are capable of forming visible colonies on a solid growth medium. To calculate the CFU in cells/mL of the original sample based on the given information, we need to consider the dilution factor and the number of colonies counted on the 10^-8 diluted petri plate. Problem: You conduct a standard plate count where 100 μL of each dilution is added to each petri plate. The petri plates diluted to 10-2 and 10-4 and 10-6 are lawns and cannot be counted. The 10-8 diluted petri plate results in 211 colonies. Calculate the CFU in cells/mL of the original sample based on these results. Answer 211,000,000,000 cells/mL in original sample or x 10^11 cells/mL in original sample Solution # of colonies counted = 211 amount of diluted sample added to the petri plate in mL = 100 μL = 0.1 mL dilution of the petri plate counted = 10^-8 Step 1: Determine the concentration of cells in the diluted sample: (# of colonies counted on the petri plate) ÷ (amount of diluted sample added to the petri plate in mL) = CFU in diluted sample (cells/mL) (211 colonies) ÷ (0.1 mL diluted sample added to petri plate) = 2,110 cells/mL in the diluted sample Step 2: Determine the concentration of cells in the original sample: (CFU in diluted sample) ÷ (dilution of the petri plate counted) = CFU in original sample (cells/mL) (2,110 cells/mL in diluted sample) ÷ (10^-8 dilution of the petri plate counted) = 211,000,000,000 cells/mL in original sample = x 10^11 cells/mL in original sample CFU = 211,000,000,000 cells/mL in original sample or x 10^11 cells/mL in original sample