What broth is used during the presumptive stage of the MPN test for total coliform?

The presumptive stage of the Most Probable Number (MPN) test for total coliforms utilizes Lauryl Tryptose Broth as it creates a selective environment that favors the growth of coliform bacteria while inhibiting the growth of competing organisms. This nutrient-rich broth contains tryptose, which provides amino acids, and lauryl sulfate, which acts as an inhibitory agent against many non-coliform organisms, ensuring that any gas production indicating coliform presence is more reliably attributed to coliform bacteria.

In this stage, the fermentation of lactose present in the broth generates gas (indicated by a gas bubble in a Durham tube), which signifies the potential presence of coliforms. This makes Lauryl Tryptose Broth the appropriate choice for monitoring water quality and determining the presence of these bacteria, which are indicators of possible fecal contamination and the safety of drinking water.

Other broths listed, such as nutrient broth, vegetable broth, and yeast extract broth, do not possess the specific selective properties necessary for the MPN test, which diminishes their effectiveness in this particular analytical context.