A student prepares a dilution series of a bacterial culture

A student prepares a dilution series of a bacterial culture. (Dilutions series are a 1/10 dilution.) They plate 0.10 ml from each tube in the on separate agar plates. After incubation, 300 colonies were observed on the fourth plate in the dilution series. What was the concentration of viable cells in the original culture?

PLEASE EXPLAIN

Solution

For solving questions of this type, you should calculate the total dilution of the bacterial culture and the colony forming unit/ml (CFU/ml). The CFU/ml of the original bacterial culture is the concentration of viable bacterial cells in that culture prior to dilution.

Serial dilution

The dilution series are a 1/10 dilution. Therefore each testtube dilution increases by 10 fold.

Therefore the dilution fold of the 4^th test tube = 10⁴ (This is the dilution factor)

Number of colonies formed on the Fourth plate = 300

Volume of culture plated =0.10 ml

Therefore, the CFU/ml of the original plate = (No. of colonies x Dilution factor)/ volume of culture plated

= (300 x 10⁴) /0.1

= 3 x 10⁷

Therefore, the concentration of viable cells in the original culture is 3 x 10⁷/ml