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

A. Escherichia coli (E. coli)
B. Salmonella enterica
C. Klebsiella pneumoniae
D. Proteus mirabilis

**Core Concept:** Enterobacteriaceae is a family of Gram-negative, rod-shaped bacteria that belong to the Enterobacterales order within the family Enterobacteriaceae. Most Enterobacteriaceae members are motile, meaning they have flagella for movement. However, there are exceptions to this rule.

**Why the Correct Answer is Right:** Proteus mirabilis is a motile Enterobacteriaceae member due to its ability to produce urease enzyme, which helps it to adhere to surfaces and move using urease-powered flagella. In contrast, Escherichia coli (E. coli), Salmonella enterica, and Klebsiella pneumoniae are all non-motile Enterobacteriaceae due to their lack of flagella.

**Why Each Wrong Option is Incorrect:**

A. E. coli is a non-motile Enterobacteriaceae member because it lacks the genes required for flagella synthesis.

B. Salmonella enterica is a non-motile Enterobacteriaceae member because it also lacks the genes responsible for flagella production.

C. Klebsiella pneumoniae is a non-motile Enterobacteriaceae member due to the absence of flagella-related genes, hilA and fliC, which are necessary for flagella synthesis.

**Clinical Pearl:** The distinction between motile and non-motile Enterobacteriaceae members is crucial for accurate diagnosis and treatment. For example, motile E. coli is often associated with urinary tract infections, while non-motile E. coli is commonly found in bloodstream infections. In contrast, Salmonella is primarily associated with food poisoning and invasive infections. Motile K. pneumoniae is commonly found in community-acquired pneumonia, while non-motile K. pneumoniae is more commonly involved in bloodstream infections.