Which one of the following bacteria is classified as facultative anaerobe ?
**Core Concept**
Facultative anaerobes are bacteria that can grow in the presence or absence of oxygen, with oxygen being optional for their growth and survival. They possess mechanisms to switch between aerobic and anaerobic metabolism, depending on the oxygen availability in their environment.
**Why the Correct Answer is Right**
The correct answer is a type of bacteria that can grow both in aerobic and anaerobic conditions. Facultative anaerobes have a versatile metabolism that allows them to utilize oxygen when available, but can also survive without it by switching to anaerobic pathways. For example, they can use the enzyme cytochrome c oxidase in the presence of oxygen to generate energy, but can also employ alternative electron acceptors, such as fumarate or nitrate, in its absence. This adaptability is a hallmark of facultative anaerobes.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because obligate anaerobes, such as Clostridium difficile, cannot grow in the presence of oxygen and are inhibited by its presence.
**Option B:** This option is incorrect because obligate aerobes, such as Pseudomonas aeruginosa, cannot grow in the absence of oxygen and require its presence for survival.
**Option C:** This option is incorrect because strict anaerobes, such as Bacteroides fragilis, cannot grow in the presence of oxygen and are inhibited by its presence.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that facultative anaerobes can cause a wide range of infections, from skin and soft tissue infections to life-threatening diseases like bacteremia and meningitis. This adaptability makes them notorious pathogens in both community and hospital settings.
**Correct Answer: C. Escherichia coli**