In a set of thioglycollate broth tubes, which tube would show the expected growth pattern for a facultative anaerobe?

• A. Growth throughout the tube, but denser at the top
• B. Even growth only in the middle of the tube
• C. Growth only at the top of the tube
• D. Growth only at the bottom of the tube

Answer: (A)

Verified step by step guidance

1. Understand the purpose of thioglycollate broth: it is a medium used to determine the oxygen requirements of microorganisms by creating an oxygen gradient from the top (high oxygen) to the bottom (low or no oxygen).
2. Recall the oxygen preferences of different types of bacteria: obligate aerobes grow only at the top where oxygen is present, obligate anaerobes grow only at the bottom where oxygen is absent, facultative anaerobes can grow throughout but prefer oxygen and thus grow denser at the top, and microaerophiles grow in the middle where oxygen is reduced.
3. Identify the growth pattern for facultative anaerobes: since they can use oxygen but also grow without it, they will grow throughout the tube but show denser growth near the top where oxygen is more abundant.
4. Compare the given options with the expected growth patterns: 'Growth throughout the tube, but denser at the top' matches the facultative anaerobe pattern.
5. Conclude that the tube showing growth throughout but denser at the top is the correct representation of facultative anaerobe growth in thioglycollate broth.