Which of the following bacteria do not invade host tissue cell?
**Core Concept**
The question is testing the ability to distinguish between different types of bacterial pathogenicity, specifically the ability to invade host tissue cells. Invasion of host cells is a key virulence mechanism employed by certain bacteria to establish infection.
**Why the Correct Answer is Right**
Bacteria that invade host tissue cells, such as Shigella flexneri and Salmonella Typhi, use their type III secretion system to inject effector proteins into the host cell, leading to cell death and tissue damage. This process requires the bacteria to interact with and manipulate the host cell membrane and cytoskeleton.
**Why Each Wrong Option is Incorrect**
**Option A:** While some strains of E. coli can invade host cells, others, such as E. coli K-12, are non-invasive and do not possess the necessary virulence factors.
**Option B:** Shigella flexneri invades host cells, using its type III secretion system to inject effector proteins.
**Option C:** Vibrio cholerae does not invade host cells; instead, it adheres to the intestinal mucosa and produces cholera toxin, leading to fluid secretion and diarrhea.
**Clinical Pearl / High-Yield Fact**
Non-invasive bacteria, such as E. coli K-12 and Vibrio cholerae, can still cause significant disease through other mechanisms, such as toxin production and adherence to host tissues.
**Correct Answer: C. Vibrio cholerae is a non-invasive bacterium that causes disease through toxin production and adherence to host tissues.**