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Calculating CFU/g

About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.

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Calculating CFU/g

Postby SWEngineer » Fri Nov 25, 2011 10:29 am

If we have a 5 gram food sample that was homogenised in sterile Ringer's solution to give a total volume of 10 cm3. After that, if we serially dilute where 0.1 cm3 aliquots of a number of dilutions are spread plated on duplicate nutrient agar plates, and after incubation, we get for example the following colony counts in dilution 10-3:

Dilution Replicate (1) Replicate (2)
10-3 315 306


I know how to calculate the CFU/cm3 of the homogenate. But, how do we calculate the average CFU/g of food sample?

Thanks.
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Postby JackBean » Fri Nov 25, 2011 11:36 am

if 5 grams give 10 cm3, what is the factor to recalculate CFU/cm3 to CFU/g? :roll:
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Re:

Postby SWEngineer » Sat Nov 26, 2011 11:27 am

JackBean wrote:if 5 grams give 10 cm3, what is the factor to recalculate CFU/cm3 to CFU/g? :roll:


As I understood after asking, after calculating CFU/cm^3. We now say that this is for (5g/10cm^3), that is for 1/2 dilution, but here, we have to find it as a whole and not as a dilution.

So, it will be: result of CFU/cm^3 * 1/0.5 = CFU/g of food sample
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