Oxygen requirements A microorganism that grows best in a low-oxygen environment (below atmospheric O₂, often requiring 2–10% O₂) is called a(n):

Introduction / Context:
Oxygen tolerance classes determine where microbes thrive and how to culture them. Some organisms require oxygen but at lower levels than are present in the atmosphere. Identifying this group is essential for diagnostics (e.g., Campylobacter, Helicobacter) and food microbiology.

Given Data / Assumptions:

• Atmospheric oxygen is ~21% O₂.
• Some microbes are harmed by high O₂ but still need small amounts.
• Definitions distinguish obligate aerobes, obligate anaerobes, facultatives, and microaerophiles.

Concept / Approach:
Microaerophiles require oxygen for respiration but at concentrations lower than atmospheric (typically 2–10% O₂) and may need elevated CO₂. They possess limited defenses against reactive oxygen species, so high O₂ is inhibitory. This differentiates them from obligate aerobes (need ~21% O₂), obligate anaerobes (cannot tolerate O₂), and facultative anaerobes (grow with or without O₂).

Step-by-Step Solution:
Parse the definition: “best in a low-oxygen environment.”Match to category that requires O₂ but below atmospheric levels.Select 'Microaerophile' as the correct term.

Verification / Alternative check:
Microaerophilic incubation systems (gas packs or controlled incubators) adjust O₂ to ~5% for organisms like Campylobacter jejuni, confirming the definition.

Why Other Options Are Wrong:

• Aerobe/obligate aerobe: prefer full atmospheric oxygen.
• Anaerobe: harmed by O₂.
• Facultative anaerobe: can grow with or without O₂; not specifically “best at low O₂.”

Common Pitfalls:
Confusing microaerophiles with aerotolerant anaerobes, which do not use O₂ but tolerate it.

Final Answer:
Microaerophile.