You culture an inoculum of bacteria using the pour plate method, and start by mi

Question

You culture an inoculum of bacteria using the pour plate method, and start by mixing 1 mL of the sample into 9 mLs of broth medium in a tube (tube 1). You repeat this process to carry out serial 1/10 dilutions, each time mixing 1 mL of the diluted bacteria with 9 mLs of medium (tubes 2, 3, 4 and 5), and then pouring a plate. You then incubate the plates for 24 hours, and count the resulting colonies on each plate. If you obtain 5 colonies on plate 1, prepared using the 1st tube, how many bacteria were present in your original sample?

Explanation / Answer

The given serial dilution is,

Tube 1 – 1: 10 dilution --> 5 colonies

Tube 2 – 1: 100 dilution

Tube 3 – 1: 1000 dilution

Tube 4 – 1: 10^4 dilution

Tube 5 – 1: 10^5 dilution

It is given that the colonies on plate 1, prepared using the 1st tube were 5, which is a 10 times diluted sample. Thus, the original sample contains the 10 times more number of bacteria, which is, 10*5 = 50 number of bacteria in 1 mL.

It can be mathematically presented as given below:

Colony forming units (CFU)/mL (from plate C) = (no. of colonies x dilution factor) / volume of culture plate

= 5*10/1 = 50 number of bacteria

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