What is considered presumptive evidence of the presence of coliform in the MPN test?

In the context of the Most Probable Number (MPN) test, presumptive evidence of the presence of coliform bacteria is indicated by the production of gas as a result of fermentation in the culture media. When coliforms metabolize lactose, they produce acid and gas, which is a key indicator of their presence. This gas production is observed in the form of bubbles in the Durham tube or within the culture broth itself. The presence of gas is a direct result of coliform fermentation, which is a distinctive characteristic of these bacteria.

While color changes in the culture media and turbidity can provide some insight into microbial growth, they are not specific indicators of coliform presence. Color changes might indicate acid production, but that could be from other non-coliform bacteria as well. Similarly, turbidity indicates growth but does not confirm that coliforms are the specific organisms responsible for that growth. The metallic sheen on colonies is often associated with specific types of other bacteria, such as E. coli, on certain selective media, but it does not directly pertain to the MPN test or coliform identification methodology.

Thus, the production of gas from fermentation is recognized as presumptive evidence indicating that coliform organisms are present in the tested sample.