A sample has between 33x 10 4 and 33x 10 6 CFUmL Devise a co

A sample has between 3.3x 10 4 and 3.3x 10 6 CFU/mL. Devise a complete but efficient (that is, no extra plates!) dilution scheme that will ensure getting a countable plate.

Solution

It\'s generally regarded that around 30 to 300 colonies is a \'countable\' plate.
A series of 1 in 10 dilutions is usual, using say, 0.1 mL of \'sample\' plus 0.9 mL of
diluent. Then transfer 0.1 mL from appropriate tubes to plates.

For example, the sample range is 3.3*10^4 to 3.3*10^6 CFU/mL. This is diluted
1 in 10 in tube 1, so the range is now 3.3*10^3 to 3.3*10^5 CFU/mL, which is
actually 3.3*10^2 to 3.3*10^4 CFU per 0.1 mL, because 0.1 mL is put onto plates.

Tube 1 : 0.1 mL of sample + 0.9 mL of diluent = 33000 to 3300000 CFU/0.1 mL.
Tube 2 : 0.1 mL from tube 1 + 0.9 mL diluent = 3300 to 330000 CFU/0.1 mL.
Tube 3 : 0.1 mL from tube 2 + 0.9 mL diluent = 330 to 33000 CFU/0.1 mL.
Tube 4 : 0.1 mL from tube 3 + 0.9 mL diluent = 33 to 3300 CFU/0.1 mL.
Tube 5 : 0.1 mL from tube 4 + 0.9 mL diluent = 3.3 to 330 CFU/0.1 mL.
Tube 6 : 0.1 mL from tube 5 + 0.9 mL diluent = 0.33 to 33 CFU/0.1 mL.

I think 33 - 330 CFU is close enough to 30 - 300 CFU for practical purposes.