I did an experiment where I evaluated the growth of colonies of a bacteria in different samples. For one of them it was not necessary to do serial dilutions. In this case my dilution factor cannot be 1. So... is it zero? Or, how would you calculate CFU/mL?
I took 100 µL of sample.
As an example, in my first replicate I obtained 99 and 107 colonies from the sample. And, according to my calculation I got 1.030E+03 CFU/mL (considering that my dilution factor is zero). Is that right?
I don't know if it's a matter of perspective, but it sounds like that's a very high count for a sample that wasn't diluted.
Thnks!!!
Your calculations are correct, if you have 100 colonies in 0.1ml then you have 1000 CFU per ml.
If in 100 µL of the sample the count is 103 CFU, then it is logical that in 1 mL it's 1030 CFU/mL.
But I'm curious to what kind of samples are you working on, because generally speaking diluting the samples in TSB or P.W or even a buffer solution does help the recovery of microorganisms before cultivation.
Dilution factor is 1 - it can't be zero, zero would give you a cfu/ml as zero
Suhil Suliman
It's common in industrial application (drug, cosmetic, etc.) to dilute in a buffer or broth matrix - e.g. USP 61.
The recovery as "high" or not would depend on the sample and we don't know that.
Your average colonies without dilution is 103 per 100 micro litre. So, the cfu will be 1030 cfu per ml which can be written
as 1.03 x 10³ . Consider 100 micro litre is 0.1 ml. Since, you have not used any dilution factor, the question of multiplying by dilution factors does not arise and the appropriate way to express cfu per ml will be 1.03 x 10³ CFU per ml.
Thanks to everyone for your replies. I appreciate the time you took to give me your reasoning.
I think that Suhil Suliman and Rajen Baruah gave me quick and logical answers.
Regarding the answers of Phil Geis, I am not completely convinced because I did my calculations using the dilution factor of 0 and it does not give me any errors.
Also Suhil Suliman, I am doing a microencapsulation protocol for a bacteria and I would expect to find little growth in a solution i used (CaCl2), which is why I did not need to dilute my sample.
Isaac MaquedaIt
Quite obviously not zero
By the equation, zero as dilution factor could ONLY give you zero cfu/ml in the sample being tested. Errors? It gives you nothing - zero times any number is zero. Zero as dilution factor would give ZERO cfu/ml
cfu/ml = (colonies counted X dilution factor)/volume of original sample
1030 cfu/ml = (103 X 1)/0.1ml
0 cfu/ml - (103 X 0)/0.1ml
I already understand what you're telling me Phil Geis. I was confusing the concept of dilution factor. For anyone interested in understanding:
- If I didn't occupied any dilution; my dilution factor is 1 (100 = 1)
- If I used the 10-1 dilution; my dilution factor is 10 (101 = 10)
- If I used the 10-2 dilution; my dilution factor is 100 (102 = 100) and so on...
Is that what you meant Phil Geis ?
If that's the case, there are two formulas and both give the same result:
A) UFC/mL = {(Colonies)(Dilution Factor)} / Plated volume
B) UFC/mL = (Colonies)[1/Dilution used)] / Plated volume
Example:
A) (103)(100) / 0.1 = 1030
B) (103)(1/100) / 0.1 ) = 1030
If I had used the 1st dilution the results would be these:
A) (103)(101) / 0.1 = 10300
B) (103)(1/10-1) / 0.1 ) = 10300
Thank you all!