The function of the tetrazolium salt (TTC) in motility test medium is to:

• A. inhibit the growth of non-motile bacteria
• B. solidify the medium to prevent bacterial movement
• C. provide nutrients required for bacterial growth
• D. act as an indicator by turning red where bacteria are metabolically active

Answer: (D)

Verified step by step guidance

1. Understand the purpose of the motility test medium, which is designed to determine whether bacteria can move (are motile) or not.
2. Recognize that tetrazolium salt (TTC) is added to the medium as a color indicator rather than as a nutrient or solidifying agent.
3. Know that TTC is colorless in its oxidized form but turns red when reduced by bacterial metabolic activity, indicating areas where bacteria are actively respiring.
4. Realize that motile bacteria will move away from the stab line, reducing TTC in a wider area and causing a diffuse red coloration, while non-motile bacteria will only reduce TTC along the stab line.
5. Conclude that the function of TTC in the motility test medium is to act as an indicator by turning red where bacteria are metabolically active, helping visualize bacterial movement.