Microbial oxygen relationships: What does the term “aerotolerant anaerobe” precisely mean in bacterial physiology and cultivation practice?

Introduction / Context: Understanding microbial oxygen requirements is fundamental for culturing, diagnosing infections, and designing bioprocesses. Key categories include obligate aerobes, obligate anaerobes, facultative anaerobes, microaerophiles, and aerotolerant anaerobes. Each category reflects enzyme complements for handling reactive oxygen species and terminal electron acceptor usage.

Given Data / Assumptions:

• The definition sought is specifically for “aerotolerant anaerobe.”
• Typical examples include many lactic acid bacteria (e.g., some Lactobacillus spp.).
• We assume standard laboratory oxygen exposure conditions.

Concept / Approach: Aerotolerant anaerobes do not use oxygen as a terminal electron acceptor and therefore do not gain energy via aerobic respiration. However, they possess detoxification systems (e.g., superoxide dismutase, peroxidases) that allow survival and even growth in the presence of oxygen. This contrasts with strict anaerobes (oxygen is lethal), facultative anaerobes (can switch to aerobic respiration when oxygen is present), and microaerophiles (require reduced oxygen tension).

Step-by-Step Solution: List the main oxygen relationship categories and their defining traits. Match “aerotolerant anaerobe” to organisms that do not respire with oxygen but tolerate it. Exclude options describing strict anaerobes, microaerophiles, or facultative anaerobes. Select the statement that explicitly states “doesn’t use oxygen but can grow in its presence.”

Verification / Alternative check: Growth patterns in thioglycollate medium show aerotolerant anaerobes distributed throughout the tube, unlike strictly top (aerobes) or bottom (strict anaerobes) growth patterns.

Why Other Options Are Wrong: Strict anaerobes are oxygen-sensitive (option b); microaerophiles need low oxygen (option c); facultative anaerobes use oxygen when present (option d). Option e is vague and not the accepted definition.

Common Pitfalls: Confusing aerotolerance with facultative respiration; tolerance does not imply using oxygen for ATP generation.

Final Answer: Does not use oxygen for energy metabolism but can grow in the presence of oxygen.