If 0.150 mL of 10^-7.5 dilution yielded an average of 256 colonies/plate, calculate the number of bacteria per mL of the original culture. Show all calculations. Solution The number of colony-forming units (CFU) = [no of cells]/ [volume of dilution x volume of diluted sample added to the plate] Now CFU per ml of dilution is 256 colonies divided by 0.150 ml plated = 1706.7 colony- forming units/ml of dilution. Dividing this number by the 10-7.5 dilution = 1706.7 / 10-7.5 = 1706.7 x 107.5 colony-forming units/ml of original culture sample.