Describe the principle of MPN for enumerating bacteria from a natural sample.

MNP stands for the most probable number which is used for finding out the concentration of viable bacteria in a sample. It is commonly used to check the quality of water and its safety. Usually, the presence of fecal coliform bacteria is checked in this test.

Principle of MNP: The sample which is to be tested is diluted serially and then inoculated into a broth medium containing lactose. If coliforms are present in the tube than it will consume lactose and release acid and gas.

Color change represents acid production and gas trapped in the Durham tube represents the gas production by bacteria. Then the number of tubes at each dilution that showed positive results are compared with standards to determine the most probable number of the bacteria.