The presumptive evidence of the presence of coliform in the MPN test is:

The presence of gas from the fermentation of the media is considered presumptive evidence of coliform bacteria in the Most Probable Number (MPN) test. Coliforms, particularly fecal coliforms like Escherichia coli, can ferment lactose, which generates gas as a byproduct of this fermentation process.

When lactose is fermented in the culture media used for the MPN test, it typically occurs in the presence of specific nutrients that encourage the growth of coliforms. As coliforms grow and metabolize lactose, the production of gas is observed, often indicated by the formation of gas bubbles in the Durham tube (a small inverted vial placed in the culture). If gas is detected, it serves as a strong indicator that coliform bacteria are present, supporting the conclusion that water or food samples may be contaminated.

While other indicators, such as color change in the culture media or turbidity, can also provide some information about microbial activity, they are not definitive or exclusive indicators of coliform presence in the same way that gas production is. The metallic sheen on colonies is typically associated with specific media used for detecting E. coli but does not apply to the broader context of the MPN test, which encompasses all coliforms.