Calculating the Original Concentration of Bacteria

[higherme]

I am not really sure how to find the original concentration of bacteria for my experiment.

This is what i did:
I made serial dilutions of 10^-2, 10^-4,10^-6, 10^-8 with stock bacteria.
From 10^-4 dilution, I took out 2ml and inoculated into 20g potatoes + 180g buffer. Potatoes + buffer + inoculum were homogenized. Then I took 0.1 ml from the homogenate and plate onto agar.

If i want to find the original concentration in my stock bacteria, is this what I do:

plate count = 30 colonies

(30 x 10^4) / (10)(2)
= 1.45x10^6 cfu/ml

can someone check if my calculations are correct? Thanks!

 

[higherme]

the equation should actually read:

(30 x 10^4) / (0.1)(2)
= 1.45 x 10^6 cfu/ml