On two sets of subsequent duplicate plates you have generate

On two sets of subsequent duplicate plates, you have generated the following counts:

      10^-2 = 99 and 105

      10^-3 = 11 and 20

What is your ONE final reported count with correct units for this sample of pasteurized milk?

Solution

10^-2 is 1:100 dilution

10^-3 is 1:1000 dilution

Take the average for the duplicate plates at each dilution

Use the formula

B = N/D

N= number of bacteria

D= Dilution factor

1. B = 102/10^-2 = 1.02 x 10⁴ CFU/ml

2. B = 15.5 / 10^-3 = 1.5 x 10⁴ CFU/ml