Fundamentals of Microbiology (2+2)

Observation, Counting and Reporting

Observation, Counting and Reporting:

            After 48 h of incubation, the colonies that are developed in each plate are counted using a Qubec colony counter. Colonies are also counted manually by touching the base plate, where each colony could be seen, with a marker tip. The colony counts of duplicate plates should agree within ten per cent limit and the counts between decimal dilutions should agree decimally. The count in the given sample is calculated using the following relation.

APC or TPC / ml of the given sample = Average count x reciprocal of the dilution  x  Correction factor

(The correction factor is 10 for spread plate technique if the volume of inoculum is 0.1 ml or 2 if the volume of inoculum is 0.5 ml)

The APC or TPC is expressed as cfu / ml i.e., colony forming units per ml of the sample. For the purpose of calculating, only those plate counts that fall between 30 and 300 are statistically acceptable. USFDA has suggested considering counts between 25 and 250 as statistically acceptable.

Tabulation of Results

Sample	Colony Counts in Dilutions			Cfu / ml
R1 R2

Result and Inference