In the membrane filtration procedure using m-Endo agar, what color are the coliform colonies?

In the membrane filtration procedure utilizing m-Endo agar, the coliform colonies exhibit a characteristic coloration of pink to dark red with a metallic sheen. This coloration is a result of the specific formulation of m-Endo agar that contains various nutrients and dyes that promote the growth of coliform bacteria while allowing for easy identification.

The pink to dark red color indicates the fermentation of lactose by coliforms, producing acid that reacts with the indicators in the agar, leading to this distinct coloration. The metallic sheen observed on the surface of these colonies is particularly indicative of the presence of coliform bacteria, primarily E. coli. This specific coloration and sheen provide microbiologists with a reliable visual cue for the presence of coliforms in water samples, which is critical for assessing water quality and safety.

Other potential colors mentioned in the choices would not yield the same level of specificity or recognition for coliforms, thus making the pink to dark red with metallic sheen the definitive choice for identifying these bacteria in the membrane filtration process.