## Core Concept
Facultative anaerobes are bacteria that can live and grow in both the presence and absence of oxygen. They have the ability to switch between aerobic respiration and fermentation or anaerobic respiration, depending on the availability of oxygen.

## Why the Correct Answer is Right
The correct answer, *Escherichia coli*, is a classic example of a facultative anaerobe. It can grow in the presence of oxygen by performing aerobic respiration, producing more ATP, and can also survive in low-oxygen environments by switching to fermentation or anaerobic respiration. This adaptability allows *E. coli* to thrive in various environments, including the human gut.

## Why Each Wrong Option is Incorrect
* **Option A:** *Clostridium perfringens* is an obligate anaerobe, meaning it strictly requires an absence of oxygen to grow. It cannot survive in the presence of oxygen.
* **Option B:** *Staphylococcus aureus* can grow in the presence or absence of oxygen but is more accurately described as a microaerophile or aerotolerant. However, the term facultative anaerobe is often broadly applied to bacteria like *E. coli*, which more precisely fit the definition.
* **Option D:** *Bacillus subtilis* is typically considered a facultative anaerobe or aerobe; however, among the given options, *E. coli* is the most quintessential example.

## Clinical Pearl / High-Yield Fact
A key point to remember is that facultative anaerobes can cause a wide range of infections, depending on their location and the host's condition. *E. coli*, for instance, is a common cause of urinary tract infections, pneumonia, and gastrointestinal infections.

## Correct Answer: C. Escherichia coli