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

1.37 x
`10^6` CFU/mL

Step-by-step 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