Hi,
I just did some calculation base on what weredoing in our lab. Please check if Ididthe right calculation.
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