CFU per microlitre bacteria - How do i calculate it? (Jan/08/2010 )

Right my Professor at Uni has set me a problem. he wants me to express bacterial counts in terms of colony forming units per microlitre (ul).
We have plate counts and we have irradiated bacteria namely S.poona, E.coli and S.aurues to pulsed and non pulsed irradiation. I was initially working from the dilutions I used i.e 10-5, 10-6 and 10-7, since these gave us counts below 300CFU and now he tell me he wants me to express results in the CFU/ul. Fashion.

Can anybody help?
There is a mars bar in it if we ever meet up at a conference !!

Eum,

You just need to count the colonies on your plate and you know how much microliter you used to put in on the plate....
So you can calculate how much CFU it is per microliter (if you added 1 microliter per plate) and thus also how much per vial or sample.

To express the cell density in terms of CFU/uL you need three things: your plate count, the volume plated, and the dilution factor.

Here's a simple example: you plate 100 uL at a dilution of 10^-6 and you get 100 colonies. That means in the original fluid, there were 10^6 as many CFU as in your sample, so you multply the count by the dilution factor: 100 * 10^6 = 10^8. But that is 10^8 in 100 uL so you divide by 100, and get 10^6 CFU/uL. That's the general formula.

Mathematically it's N = P/D/V where N = cell density, P = number of colonies on plate, V = plated volume, and D is the dilution (remember, the exponent is negative so dividing by 10^-3 is the same as multiplying by 10^3). I would put this into a spreadsheet and automate the process.

Thank you for your help!!!