The original concentration of bacteria in a sample is 5.5x10^6 CFU/ml. Assuming you are going to plate 0.1 ml of the dilution onto your agar plate, which dilution factor should you use to give you a countable number of colonies? *my Lab teacher doesn't teach us any of this stuff she just expects us to know it. I do not understand it at all. Very confusing. I need taught to me step by step please and thank you.

Question

The original concentration of bacteria in a sample is 5.5x10^6 CFU/ml. Assuming you are going to plate 0.1 ml of the dilution onto your agar plate, which dilution factor should you use to give you a countable number of colonies?   *my Lab teacher doesn't teach us any of this stuff she just expects us to know it. I do not understand it at all. Very confusing. I need taught to me step by step please and thank you.

anonymous · Answer

Don't worry - I'll try ands explain it as best I can: the question could be put a different way, for example how much should you dilute the sample so you get 55 colonies on a plate. The value for the sample is 5.5x10^6CFU per ml, so 0.1ml would have 5.5x10^5 colony forming units. This is the same as saying that if you dilute the sample by a factor of 1000(or10^3) you would have 5.5x10^2 or 550 colonies on the plate. So if you diluted by 10,000 (10^4) you would have about 50 colonies per plate. I wouldn't mind counting about 50 but 500 seems too much.

In case you're having trouble with the numbers, to divide 10^5 by 10^2 you subtract the 2 from the 5 to get 3, so the answer is 10^3 or 1000. If you get sceintific notation then things will get a lot easier. Best wishes, E