you are handed a broth tube of a microbe and asked to perform a serial dilution

ID: 54099 • Letter: Y

Question

you are handed a broth tube of a microbe and asked to perform a serial dilution and spread plalating to ascertain the CFU/m: value for the initial culture. You are given a set of 6 saline tubes, each containing 9 mL. You perform serial dilutions from the orginal culture tubes (call this tube #0) in this set of tubes, transferring 1 mL of fluid from each in series to the next tube, mixing before each transfer. You then plate 0.1 mL of fluid form each in onto two separate TSA plates from tubes #4-6. the plates are incubated overnight at 37C, and you count the colonies on the plates the next day. on the two plates from tuve #4 you count 453 and 415 colonies. On the two plates from Tube #5, you count 42 and 50 colonies. on the two plates form tube #6, you count 3 and 5 colonies. What is the CFU/mL value for the original cuture?

Explanation / Answer

Dilution

CFU per 0.1 ml (average of the two plates)

CFU/ml

Cells in the original sample/ml

10-6

4.0 × 107

10-5

460

4.6 × 107

10-4

434

4340

4.34 × 107

The above results show that in the original sample, there are approximately 4.34 × 107 cells/ml