ENUMERATION OF COLIFORM BACTERIA Standard multiple tube fermentation technique (determination of the most probable number, MPN)
Principle The standard tests are
Confirmed test and
Graduated amount of food are transferred to series of fermentation tubes containing lactose broth or lauryl sulphate tryptose broth of proper strength. It is usual practice to inoculate to five fermentation tubes
The tubes are incubated at 35±0.50c and examined at the end of 24±2 hours. Tubes ssowed no gas productions at the end of 24+2 hours are reincubated and examined at the end of 48±3hrs.
The formation of gas in any of the tubes within 48±3 hours, regardless of the amount, constitutes a positive presumptive test.
The absence of gas formation within that period constitutes a negative test, and no further tests need be performed.