Which of the following test tubes shows the growth pattern of an obligate aerobe in thioglycollate broth?

• A. Growth only at the bottom of the tube
• B. Uniform growth throughout the tube
• C. Growth concentrated in the middle of the tube
• D. Growth only at the top of the tube

Answer: (D)

Verified step by step guidance

1. Understand the purpose of thioglycollate broth: it is a differential medium used to determine the oxygen requirements of microorganisms by creating an oxygen gradient in the tube, with high oxygen concentration at the top and low or no oxygen at the bottom.
2. Recall the oxygen requirements of an obligate aerobe: these organisms require oxygen for growth and will only grow where oxygen is present, which is at the top of the thioglycollate broth tube.
3. Analyze the growth patterns: growth only at the bottom indicates obligate anaerobes (no oxygen), uniform growth throughout indicates facultative anaerobes (can grow with or without oxygen), and growth concentrated in the middle suggests microaerophiles (require reduced oxygen levels).
4. Match the growth pattern of an obligate aerobe to the correct description: since obligate aerobes need oxygen, their growth will be limited to the top of the tube where oxygen is abundant.
5. Conclude that the test tube showing growth only at the top corresponds to the obligate aerobe's growth pattern in thioglycollate broth.