The term facultative anaerobe refers to which type of organism in relation to oxygen use?

Introduction / Context:
Microorganisms show different relationships to oxygen, which influences where they can live and how they are cultivated in the laboratory. Terms such as obligate aerobe, obligate anaerobe, facultative anaerobe, microaerophile, and aerotolerant anaerobe describe these patterns. This question focuses on the definition of a facultative anaerobe, which is a very common and medically important category of bacteria and yeasts.

Given Data / Assumptions:

Oxygen tolerance and use can vary greatly between organisms.
A facultative anaerobe by definition can grow under more than one oxygen condition.
We assume standard microbiology terminology for oxygen relationships.

Concept / Approach:
A facultative anaerobe is an organism that can use oxygen for aerobic respiration when oxygen is available, which generally allows it to grow more efficiently and produce more ATP. However, when oxygen is absent, the same organism can switch to anaerobic respiration or fermentation pathways and continue to grow. Examples include many bacteria such as Escherichia coli and many yeasts. This flexibility gives them a wide ecological range. This is different from microaerophiles that require low oxygen levels, obligate anaerobes that are killed by oxygen, or aerotolerant anaerobes that do not use oxygen but can survive its presence.

Step-by-Step Solution:
Step 1: Recall that facultative means optional or capable of functioning under different conditions. Step 2: Combine this with anaerobe, which refers to growth in the absence of oxygen, to understand that the organism can grow with or without oxygen. Step 3: Identify the option that explicitly states that the organism can use oxygen when present but also grow without it. Step 4: Choose that option as the correct definition of a facultative anaerobe.

Verification / Alternative check:
In culture tubes such as thioglycollate medium, facultative anaerobes show growth throughout the tube but often grow best near the surface where oxygen is higher. This growth pattern shows that they can thrive with oxygen yet still grow without it, supporting the definition given in the correct option.

Why Other Options Are Wrong:
Requires less oxygen than air: This describes microaerophiles, which need reduced oxygen levels but cannot grow well without oxygen.
Prefers to grow without oxygen: This more closely resembles aerotolerant anaerobes, which do not use oxygen even if present.
Killed by oxygen: This is the definition of obligate anaerobes, which cannot survive in the presence of oxygen.

Common Pitfalls:
Students sometimes confuse facultative anaerobes with aerotolerant anaerobes because both can grow without oxygen. The key difference is that facultative anaerobes use oxygen when available for higher energy yield, whereas aerotolerant organisms do not shift to aerobic respiration even if oxygen is present. Remembering real life examples like E. coli, which grows in both gut and oxygenated environments, helps solidify the concept.

Final Answer:
A facultative anaerobe is an organism that can use oxygen when it is present but can also grow without oxygen.