Final answer:
Tetrazolium salt (TCC) acts as an indicator of bacterial respiration in a motility test agar, forming a red formazan compound that signifies bacterial activity and helps in determining motility.
Step-by-step explanation:
The purpose of tetrazolium salt (TCC) in a motility test agar is to act as an indicator of bacterial respiration. When bacteria metabolize the TCC, it forms an insoluble red formazan compound. This indicates that the bacteria are metabolically active, providing an indirect measure of cell viability and motility. Bacteria that are motile will move away from the stab line, spreading the red formazan throughout the agar and thereby creating a cloudy or diffused appearance. Non-motile bacteria will leave the area near the stab line clear, with no diffusion of the red color. Therefore, TCC helps to visually determine the motility of bacteria in these assays.