**Question:** All the following are non-motile Enterobacteriaceae members except

A. Escherichia coli (E. coli)
B. Salmonella enterica
C. Klebsiella pneumoniae
D. Enterobacter cloacae

**Correct Answer:** **Option A: Escherichia coli (E. coli)**

**Core Concept:** Enterobacteriaceae is a family of gram-negative, rod-shaped, and aerobic or facultatively anaerobic bacteria. These bacteria are commonly found in the human gastrointestinal tract and are part of the normal flora. The main characteristic that differentiates Enterobacteriaceae members is their motility. Motile Enterobacteriaceae are those that can move using flagella, while non-motile members lack this ability.

**Why the Correct Answer is Right:** Escherichia coli (E. coli) is an example of a motile Enterobacteriaceae species. It has flagella which enable its movement in the gastrointestinal tract. Since E. coli is motile, it does not fit into the category of non-motile Enterobacteriaceae members.

**Why Each Wrong Option is Incorrect:**

**Option B: Salmonella enterica**
- Salmonella is another motile Enterobacteriaceae species, making it an incorrect answer as well. It has flagella and can move, distinguishing it from non-motile members.

**Option C: Klebsiella pneumoniae**
- Klebsiella pneumoniae is a motile Enterobacteriaceae species. It has flagella and can move, contradicting the non-motile characteristic required for this question.

**Option D: Enterobacter cloacae**
- Enterobacter cloacae is another motile Enterobacteriaceae species. It possesses flagella and can move, leading to its exclusion from the list of non-motile Enterobacteriaceae members.

**Core Concept (for further understanding):**

**Non-motile Enterobacteriaceae members** are bacteria that lack the ability to move due to the absence of flagella. These organisms are typically found in the gastrointestinal tract and other parts of the human body, where they play a role as part of the normal flora. Understanding the distinction between motile and non-motile Enterobacteriaceae members is important in clinical medicine, particularly when diagnosing infections and guiding treatment decisions, as motile species can be pathogenic, while non-motile species are more likely to be part of the normal flora.

**Clinical Pearls:**

- In clinical practice, distinguishing between pathogenic and commensal Enterobacteriaceae species is crucial for appropriate diagnosis and treatment strategies. Recognizing motile species as potential pathogens and non-motile species as part of the normal flora can guide decisions on antibiotic usage and infection management.
- **Option A: Escherichia coli (E. coli)** is a non-motile Enterobacteriaceae species that resides in the gastrointestinal tract and is part of the normal flora, making it a non-motile member.