Bacterial Counts - please help!?

Question

An overnight broth culture (total volume 20 mL) was diluted in 10-fold steps, and 100 micro-Litre volumes spread-plated on TSA. What is the bacterial count in the culture if the 10^ -7 dilution yields 53 colonies? (Show calculation and give units).

Answer 1

Okay, first thing is that the sample is 20 mL and you only take a 100 microliter subsample.  So, if you get 53 colonies from 100 microliters, then you would get 10,600 from the whole 20 mL (20,000/100 *53).

Now you have to deal with the dilution.  10,600/(10^-7) = 1.06 x 10^11 bacterial cells.