What factor allows visibility of acid production by oxidizers in oxidation-fermentation (OF) medium?

The visibility of acid production by oxidizers in oxidation-fermentation (OF) medium is primarily dependent on the low peptone content in the medium. This is crucial because low peptone levels minimize any background alkaline or neutral reactions that could obscure the detection of acid production. In an OF medium, the primary carbon source is typically a sugar, such as glucose, and the peptone serves as a nitrogen source.

When microorganisms metabolize the sugar under aerobic conditions, they produce acid, leading to a pH drop. The low peptone content ensures that microbial growth is primarily fueled by the sugar rather than the peptone, allowing for a clearer indication of acid production. If the peptone concentration were high, it might produce its own degradation products, leading to confusion in interpreting the acid production.

Thus, low peptone content effectively allows for the clear visibility of any changes in pH due to acid production as a direct result of carbohydrate metabolism in the medium.