Most Probable Number (MPN):
Most Probable Number is a number, which represents the bacterial density, which is most likely to be present in the water. Different dilution of sample of water is mixed with lactose broth and incubated in test tubes for 48 hours at 37oC. The presence of acid or carbon dioxide gas in the test tubes indicates the presence of coliform bacteria. Referring to the standard statistical tables, MPN of E Coli per 100 ml of water is determined. In order to ensure that the water is safe for drinking and free from pathogenic bacteria and other organisms, it is seen that on an average not more than one coliform colony is present per 100 ml of water. The test is generally performed on 5 samples and an average of the coliform colonies taken, which should not exceed 1 per 100 ml, and results are compared with standard table. It is ensured that while taking samples of bacteriological test, sample should be collected from different points of distribution system and tested immediately after they are collected and in no case testing should be done beyond 24 hours of collection.