Facultative anaerobes – definition check: What is the correct definition of a facultative anaerobe in terms of oxygen usage and growth capability?

Introduction / Context:
Facultative anaerobes are among the most versatile microbes, thriving in diverse environments. Understanding their metabolism is crucial for infection control, fermentation, and environmental microbiology.

Given Data / Assumptions:

• Facultative anaerobes adjust metabolism between respiration and fermentation.
• They possess enzymes to detoxify reactive oxygen species.
• Classic examples include Escherichia coli and many yeasts.

Concept / Approach:
In the presence of oxygen, facultative anaerobes respire aerobically for higher ATP yield. In anoxic conditions, they switch to anaerobic respiration (alternate electron acceptors) or fermentation. This flexibility supports growth across oxygen gradients typical of natural and clinical settings.

Step-by-Step Solution:
Contrast key oxygen categories (obligate aerobe/anaerobe, microaerophile, aerotolerant). Identify the defining trait: growth with or without oxygen, using oxygen when available. Select the statement that captures this dual capability.

Verification / Alternative check:
Growth in thioglycollate medium shows a dense band at the top (where oxygen is) for facultatives, but with growth throughout the tube as well, unlike strict aerobes or anaerobes.

Why Other Options Are Wrong:
Option a defines aerotolerant anaerobes; option b/e describe obligate anaerobes; option d defines microaerophiles.

Common Pitfalls:
Assuming “facultative” means preference for anaerobiosis; many facultatives prefer oxygen when available due to energetic advantage.

Final Answer:
Uses oxygen when present but can grow without oxygen when it is absent.