The two methods for the analysis of drinking water for coliforms are as follows:- 1. MPN (Most probable number) test. 2. Membrane filtration test.
Method # 1 For MPN Three Tests are Done:
These are presumptive test, confirmed test and completed test. For presumptive test the lactose broth is inoculated and incubated for 24 to 48 hours. Production of gas indicates coliform positive in presumptive test while if the gas is not produced the coliform is absent.
Confirmed Test The samples from the lactose broth are streaked on the EMB lactose plates and are incubated for 24 hours. The typical colonies with dark centres or metallic sheen indicate positive confirmed test. The absence of colonies of coliform is the negative result.
Completed Test Select the typical colonies of coliform (with metallic sheen), inoculate on lactose broth and agar slant and incubate for 24 hours. If gas is produced it shows the presence of coliform group and the completed test is positive.
Also check the slant for Gram-negative rods (which should be present) and for absence of endospores. If both these, i.e., agar slant and lactose broth show the presence of coliform group it is a positive completed test. However, if gas is not produced it is a negative completed test and the original isolates are not coliform.
Method # 2 Membrane Filtration Test:
Some laboratories adopt membrane filtration methods to test for coliforms. The coliform colonies have a distinctive appearance showing dark centres or metallic sheen.In membrane filter test 100 ml of sample is taken and is filtered through a membrane filter unit.
The filter membrane is removed along with the coliform, sieved out on surface. The filter is placed on a pad of medium similar to the EMB medium. It is incubated for 24 hours. Appearance of four coliform colonies per 100 ml is unsatisfactory while one coliform colony per 100 ml is regarded satisfactory.