Medical microbiology – aerobic status of common gram-negative genera Among the following gram-negative bacteria, which genus is not strictly aerobic but instead grows as a facultative anaerobe?

Introduction / Context:
Knowing the oxygen requirements of major bacterial genera helps predict growth patterns, choose culture conditions, and interpret clinical microbiology results. Many gram-negative rods differ in whether they are obligate aerobes or facultative anaerobes.

Given Data / Assumptions:

• Pseudomonas and Neisseria are classic gram-negative genera associated with aerobic metabolism.
• Escherichia (for example, E. coli) commonly grows with or without oxygen.
• We are asked to identify the option that is not aerobic.

Concept / Approach:

Obligate aerobes require oxygen for respiration. Facultative anaerobes can respire aerobically when oxygen is present but switch to fermentation or anaerobic respiration when it is absent. Pseudomonas spp. are textbook obligate aerobes. Neisseria spp. are aerobic (often capnophilic). Escherichia spp. are facultative anaerobes, not strictly aerobic.

Step-by-Step Solution:

Verification / Alternative check:

Standard lab practice grows E. coli on aerobic plates and in anaerobic jars; gas production in fermentation broths further confirms facultative metabolism.

Why Other Options Are Wrong:

• Pseudomonas: obligate aerobe; incorrect.
• Neisseria: aerobic; incorrect.
• None/All of these: contradict known physiology.

Common Pitfalls:

• Confusing “can grow in air” with “obligately aerobic.” Facultative organisms are not classified as aerobic only.

Final Answer:

Escherichia