A typical coliform colony in the membrane filter method shows which characteristics?

In the membrane filter method used for water testing, a typical coliform colony exhibits characteristics that are distinct and identifiable. The presence of pink to dark red coloration with a green metallic sheen is indicative of coliform bacteria, particularly Escherichia coli, which is a primary member of the coliform group associated with fecal contamination.

This coloration is a result of the fermentation of lactose by the bacteria present in the sample, which also produces gas. The green metallic sheen is a byproduct of the metabolism processes taking place within the colony as it interacts with the selective media designed to promote the growth of coliforms, further confirming its identity. This combination of characteristics effectively distinguishes coliform colonies from other microbial colonies that may be present in the water sample, which is key for evaluating water quality.

Other options represent characteristics that may be typical of other types of bacteria or media used in different microbiological tests. They do not provide the specific coloration and sheen associated with coliforms, making them less relevant in the context of identifying these organisms in water samples.