Dilution eqution to Log calculation

[Ulysses Valdez]

Hi,

I need a help on the calculation for my dilution and after that the conversion into Log value. See below what I have done and pleased correct me if I'm wrong. I really appreciate your help.

Formula:

(sample size)+(diluent) = (total solution) then..

(sample size) / (total solution)= (original sample dilution)

then one serial dilution of 1/10...

(original sample dilution) x (diluent + 1ml concentration) = (Dilution Factor)

then one serial dilution of 1/10  again...

(Dilution Factor) x (diluent + 1ml concentration) = (Total Dilution Factor)

For example:

11g + 99ml =110ml

11g / 110ml = 0.1 or 1:10 total dilution factor

then one serial dilution of 1/10, plus 1ml concentration...

0.1 x (9ml +1ml)= 1 or 1:100 total dilution factor

then one serial dilution of 1/10, plus 1ml concentration...

1 x (9ml +1ml)= 10 or 1:1000 total dilution factor

So, now to determine counts, the equation for this is:

 CFU/mL=  Count on Plate x dilution Factor

                      Volume Plated on Petrifilm

 So, say with that same example above with the 11g into 99mL. You plated on AC Petri film and saw 5 counts.  

(5 cfu) x (.1) / (0.1 mL) = 5 cfu/ ml

So Total colony forming units = 5 *10¹ cfu/ml

 

If 1 mL of a 1:10 dilution is plated and yields no colonies, 0 CFU/gram for the original sample can never be a reported value because the entire sample was not tested. By convention, the count is reported as < 10 (< 1 x 10) CFU/gram.

Converting cfu/ml to Log value

For example:

Total colony forming units= 5 *10¹ cfu/ml and you want to convert it into Log Value,

Just take Log(CFU/ml)

Here, log (5 *10¹) = 1.7

 

Thanks,

Uly

Calculation for Dilution.docx