What role does Triphenyltetrazolium chloride (TTC) play in motility medium?

Triphenyltetrazolium chloride (TTC) serves as an indicator that produces a red color in the presence of bacterial activity, specifically in the context of motility testing. When bacteria metabolize and reduce TTC, which is initially colorless, red formazan products are formed as a result of the reduction process. This color change allows for a clear visual identification of motility: if the bacteria can move through the medium, they reduce TTC along their path, creating a red coloration that spreads from the original inoculation site. This provides strong visual evidence of motile bacteria, making it easier to distinguish them from non-motile strains.

In contrast, other options do not accurately represent the role of TTC. It does not act as an inhibitor for Gram-positive bacteria nor does it indicate lactose fermentation. Additionally, while TTC may have an impact on growth due to its reduction properties, its primary function in motility medium is the color indication for motility assessment.