Define the Utilization of Most Probable Number Techniques?
The MPN technique (multiple tube technique) can be used to determine the presumptive coliform count in the food. The sample is homogenized adequately and decimally diluted in saline solution or peptone water. Measured portion (e.g. 1 ml) of the solution and dilutions is transferred into multiple tubes (3 or 5) of selective and differential medium e.g. LST and incubated at 35°C for 48 hours.
Production of gas in inverted Durham tube by lactose fermentation indicates coliforms. By using standard MPN tables, as given herewith, MPN/ml or MPN/g of food samples is derived from number of positive tubes at 3 successive dilutions.The method was given by Mc Crady in 1915 and is statistically based. Three tubes or five tubes method can be used by inoculating 3 serial dilution of the sample into 9 or 15 tubes of appropriate medium respectively. The number of organisms in the original sample is determined by using standard MPN table as we have already studied in the last practical.