A yellow color on a mannitol salt agar plate indicates what?

A yellow color on a mannitol salt agar plate indicates that acidic fermentation has occurred. When bacteria that can ferment mannitol are present, they metabolize the mannitol and produce acids as a byproduct. The presence of these acids lowers the pH of the medium, causing the phenol red pH indicator in the agar to turn yellow.

This change indicates that mannitol has been hydrolyzed and metabolized into acid, which clarifies why the yellow color is associated with acidic fermentation. The correct interpretation is that the fermentation process has led to the production of acid, which is the basis for the color change observed. Thus, unlike the interpretation that mannitol has not been hydrolyzed, the yellow color clearly shows that mannitol has indeed been processed by the bacteria.