Question

You counted 620 colonies on a plate with a dilution of 10^-2. How many cells/ml were in the original sample?

Answer 1

The bacteria divide by binary fission and each bacterial cell dived and produce a lump of cells which all are derived from that single bacterial cell, this visible bacterial lump is called a colony. Counting the number of colonies gives the actual number of cells. The bacterial enumeration is done by direct microscopic count & Standard plate count methods/viable count method. The direct microscopic count uses the hemocytometer to calculate the number of bacteria in a given sample. The bacterial sample is placed on a hemocytometer and the number of cells present in the ruled lines is counted which is used to calculate a total number of cells in the sample. The main disadvantage of this method is that it calculates both live and dead cells. In Standard plate count method the sample is serially diluted in a suitable medium or 0.9% saline and a certain amount of serially diluted sample is spread on agar plates and incubated in the incubator. After incubation, the number of colonies is counted and used to count the number of bacterial cells in the sample. The total number of bacterial cells/CFU are calculated by the following formula.

Colony forming units/milliliter = (number of colonies x dilution factor) / volume of sample added to culture plate

Number of colonies                                         =          620

Dilution factor =          1/100

volume of sample added to culture plate =          1ml

Colony forming units/milliliter in original sample       = 620*1/100/1ml

= 62000

                                                                            = 62000 CFUs/ml