Help with bacterial CFU dilution!

[checki]

Hi I was wondering could anyone help me with a quick dilution question,

 

When processing sputum in a micro lab dilutions are made to pull out significant microorganisms that are present in high quantities and basically by-pass the normal flora. So basically this is the dilution made:

 

- using a 10ul loop homogenised sputum is diluted into 5ml of sterile water. ( a 10^-3 dilution?)

 

- Then a 10ul loop of the 1=^-3 dilution is inoculated from the 5ml bijou onto each agar plate.

 

- 1 colony then represents 10^5 CFU/ml

 

 

My lecturer has asked us to basically explain how we have ended up with 1 colony then represents 10^5 CFU/ml. I have tried but cant really understand how to go about it, can anyone help?

 

Thanks!

[CharonY]

How is a dilution from 10 ul to 5 ml a dilution of 10^-3 ?

To the question, what is the total dilution of the solution that you put on the plate (note that you only use 5 ul).