Final answer:
The color change in a fermentation broth is caused by the production of acidic byproducts when bacteria ferment sugars, leading to a shift in pH and the change of a pH indicator in the medium. This is often accompanied by gas production, such as CO2, which helps in the identification of bacteria in the lab.
Step-by-step explanation:
When bacteria are able to ferment a particular sugar in a carbohydrate broth, the change is caused by the production of acidic byproducts. This acidic environment triggers a change in the pH, which in turn causes a pH indicator present in the medium to change color. As an example, S. aureus is able to ferment mannitol, producing acids that result in a yellow color change in the culture medium. The production of specific types of gas, such as CO2, is also a key indicator of fermentation and can be used in laboratory settings to identify bacterial species and their metabolic capabilities, such as differentiating pathogenic from non-pathogenic E. coli strains based on their ability or inability to ferment specific sugars like sorbitol.