Question

MICROBIOLOGY

1. The student also took samples at different times during the growth to do dilution plating and get viable plate counts. Here are the results for the samples taken at hours 3 and 6:

3 hours	6 hours
10-3 dilution = too many to count	10-5 dilution = too many to count
10-4 dilution = 476 CFUs	10-6 dilution = 155 CFUs
10-5 dilution = 49 CFUs	10-7 dilution = 17 CFUs
10-6 dilution = 5 CFUs	10-8 dilution = 2 CFUs

Which dilution will give the best estimate for the 3 hour time point? Which will give the best estimate at 6 hours? Using these data, how many CFUs/ml were present in the original culture at times 3 and 6 hours? (Assume 1 ml inoculum for for each plate)

Answer 1

We generally take the dilution that give colonies between 30-300 on the plate.

For 3 hours 10-5 dilution gave 49 CFUs. So, 10-5 dilution give the best estimate at 3 hours

For 6 hours 10-6 dilution gave 155 CFUs. So, 10-5 dilution give the best estimate at 3 hours

So we will take those dilutions to calculate CFU/ml in the original sample

CFU/ml = number of colonies / volume plated * volume in ml.

For 3 hours = 49 / 10-5*1

49 * 10^5

=4.9*10^6 CFU/ml.

So number of CFUs/ml were present in the original culture at times 3 hours is 4.9*10^6 CFU/ml.

For 6 hours = 155 / 10-6*1

155 * 10^6

=1.55*10^8 CFU/ml.

So number of CFUs/ml were present in the original culture at times 6 hours is 1.55*10^8 CFU/ml.

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