Cell number from dilution factor.

[zhar2]

How can i find bacteria concentration from dilutions?
If i got 2 dilutions of 10-5 & 10-6 (volume of 0.1 ml) & 36 colonies for 10-5 & 7 for 10-6.

[JackBean]

From one you get 3.6*10^6 and from second you get 7*10^6. If you have only these two numbers, you don't have much more choices, than to make average... (but the mistake will be pretty big)

[zhar2]

So ou just multiplied colonies by dilution factor?

[JackBean]

Sure

[mkwaje]

Actually using the old method of bacterial enumeration. you have to consider only the plates containing colonies between 30-300.. For fungal plates, they adjusted it to 25-250... At these colony numbers, there is a higher chance of getting the correct CFU. Discard the plates with lower than 30 or higher than 300 since the counts you will obtain if you use the formula will just be estimates.

cfu/ml = ave. # of colonies x DF
............total volume plated


Therefore, using only the 10^-5 dilution (since it has 36 colonies [falling between 30-300]), you'll have

cfu/ml = 36 x 10^5 = 3.6 x 10^7 CFU/ml
...............0.1

I didn't even need my calculator for this...

Good luck!