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Presumptive Test

A series of lactose broth or lauryl sulphate tryptose broth fermentation tubes are inoculated with measured amounts of water and incubated at 35°C for 24 to 48 hours.

The formation of gas in the inverted vial in the fermentation tube within 48 hours indicates a positive presumptive test. Absence of gas formation at the end of 48 hours constitutes a negative presumptive test. This means that the water sample does not contain coliforms and is considered safe.