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Postby angel92 » Mon Jul 22, 2013 9:42 pm

Following a serial dilution and spread plate experiment, a student counted the following number of colonies on nutrient agar plates inoculated with 100 l of respective dilutions from an original broth culture of E. coli.
10-5 = Too many to count (TMTC) colonies
10-6 = 248 colonies
10-7 = 33 colonies
10-8 = 1 colony
1. From this data state how many colony forming units (CFU) are present in 100 l of each dilution. Remember to state the units (state units – CFU 100 (l-1)
2. How many CFU were present in 1 ml of each dilution? (state units = CFU ml-1)
3. From this data determine how many cells are present in a typical ml of the undiluted culture. (state units = CFU ml-1)
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Postby JackBean » Mon Jul 29, 2013 10:48 am

I hope they were spreading only 100 ul (mikrolitres), because spreading 100 l (litres) of culture on single plate would take ages :-D
just type the number you got:
10^-6 - 248 CFU/100ul
10^-7 - 33 CFU/100ul
10^-8 - 1 CFU/100ul
2. just ten times more obviously
3. just divide CFU/ml by dilution (e.g. 10/10^-8 = 1×10^9), so you'll get 2.48×10^9; 3.3×10^9; 1×10^9 (although I would take the last one out) and make average

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