A student makes a series of dilutions similar to experiment 15. The original culture contains 20 mL. For the first dilution, 0.1 mL is taken from the original culture and transferred to Tube A which contains 9.9 mL of water. Then 1 mL from Tube A is plated on Plate A and 0.1 mL from Tube A is plated on Plate B. After incubation, Plate A has 305 colonies and Plate B has 31 colonies. What is the total cfu in the original culture.

615000 cfu

Explanation:

Original culture contains 20 ml

0.1 ml is taken from original culture and transferred to tube A contains 9.9 ml of water making 10 ml all together.

1 ml from tube A plated on plate A yields 305 colonies

10 ml will yield 3050 colonies

0.1 ml yielded 31 colonies

1 ml of the sample will yield 310 colonies

10 ml will yield 3100 colonies assuming that the mixture was mixed thoroughly

mean number of colonies in 10 ml yielded by the both plating = ( 3050 + 3100) / 2 = 3075 colonies

0.1 ml of the original sample contains 3075 colonies

20 ml will contain = (20 × 3075 colonies) / 0.1 = 615000 cfu