Question

Taylor, a MIC 206 student needs to determine the OCD of a sample of Escherichia coli. She performed dilutions using four 9 ml dilution blanks and plated 1.0 ml from the final dilution tube. 137 colonies grew on the plate. Calculate the number of CFU/ml that should be present in the original E. Coli sample.

Answer 1

Answer:

1.37 x $10^6$ CFU/mL

Explanation:

First, the dilution factor needs to be calculated.

Since four 9 ml dilution blanks were prepared, the dilution factor that yielded 137 colonies is of $10^{-4}$.

Next is to divide the colony forming unit from the dilution by the dilution factor:

   137/$10^{-4}$ = 137 x $10^4$

In order to get the CFU/ml, divide the CFU from the dilution by the plated volume (1 mL) from the final dilution tube.

   137 x $10^4$/1 = 1.37 x $10^6$

Hence, the CFU/ml present in the original E. coli sample is 1.37 x $10^6$.

cfu/ml = (no. of colonies x dilution factor) / volume of culture plate