CFU per microlitre bacteria
Posted 08 January 2010 - 12:13 PM
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 !!
Posted 09 January 2010 - 09:26 AM
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.
If you don't know it, then ask it! Better to ask and look foolish to some than not ask and stay stupid.
Posted 17 January 2010 - 12:47 PM
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.