In a thioglycollate broth test tube, where would you expect to find the growth of a microaerophilic bacterium?

• A. Only at the very top of the tube, where oxygen concentration is highest
• B. Only at the bottom of the tube, where there is no oxygen
• C. Just below the surface of the medium, where oxygen concentration is low
• D. Evenly distributed throughout the tube

Answer: (C)

Verified step by step guidance

1. Understand the oxygen requirements of a microaerophilic bacterium: these bacteria require oxygen but at lower concentrations than atmospheric levels, meaning they thrive in environments with reduced oxygen.
2. Recall the oxygen gradient in a thioglycollate broth tube: oxygen concentration is highest at the very top of the tube (exposed to air) and decreases progressively toward the bottom, where it is anaerobic (no oxygen).
3. Identify where microaerophiles grow in this gradient: since they need oxygen but at lower levels, they will not grow at the very top (too much oxygen) or at the bottom (no oxygen).
4. Locate the zone just below the surface of the medium where oxygen concentration is reduced but still present, which provides the ideal environment for microaerophilic bacteria to grow.
5. Conclude that the growth of microaerophilic bacteria in a thioglycollate broth will be observed just below the surface of the medium, corresponding to the area with low oxygen concentration.