In Phenol Red broth, what color indicates that sugar fermentation has occurred?

In Phenol Red broth, the presence of sugar fermentation is indicated by a color change to yellow. This occurs because, during the fermentation process, the bacteria produce acid as a byproduct from the fermentation of sugars. Phenol Red is a pH indicator, which changes color in response to the acidity of the solution. When the pH drops due to acid production, the broth shifts from its original red color to yellow, signaling that fermentation has taken place.

This color change is crucial for differentiating between bacteria that can ferment specific sugars and those that cannot, making it a valuable tool in microbiology for identifying microbial metabolic capabilities.