Questions: Type III Secretion Systems and Bacterial Virulence

Question 1

A Salmonella mutant cannot assemble a functional T3SS needle but is otherwise intact. Compared to wild-type Salmonella, what would you predict when this mutant encounters intestinal epithelial cells?

• A. The mutant invades epithelial cells more rapidly by diffusing through tight junctions
• B. The mutant fails to invade epithelial cells because it cannot inject the effectors that induce actin-driven membrane ruffling and engulfment
• C. The mutant kills epithelial cells by secreting toxins into the extracellular space instead
• D. The mutant uses a T7SS as an alternative injection mechanism

Answer: (B)

Salmonella invasion of non-phagocytic epithelial cells depends entirely on T3SS-injected effectors that reorganize the host actin cytoskeleton, causing membrane ruffling that engulfs the bacterium. Without a functional needle, these effectors cannot reach the host cytoplasm, and the epithelial cell will not ingest the bacterium on its own. The mutant is non-invasive — not because it is weaker, but because it lacks the delivery mechanism for its key virulence tools.

Question 2

The T3SS is described as a 'molecular syringe.' What biological problem does this architecture solve that simpler secretion systems cannot?

• A. It allows bacteria to secrete proteins without consuming ATP
• B. It enables secretion of fully folded, native proteins directly into host cells without any unfolding
• C. It delivers effector proteins directly into the host cell cytoplasm, bypassing extracellular immune defenses and enabling direct manipulation of host signaling from inside
• D. It allows bacteria to detect host cell proximity through chemical sensing before any physical contact

Answer: (C)

Conventional secretion releases proteins into the extracellular space, where antibodies, complement, and proteases can neutralize them, and where they must bind surface receptors to have any cellular effect. The T3SS needle punctures the host membrane and deposits effectors directly in the cytoplasm — inside the cell, where host effector defenses cannot reach. This direct cytoplasmic access is what makes T3SS uniquely powerful for hijacking host cell behavior.

Question 3

The T3SS and the bacterial flagellum are structurally unrelated, representing independently evolved solutions to the problem of protein secretion.

• T. True
• F. False

Answer: False

The T3SS and flagellum share an evolutionary ancestor — both feature a basal body with ring structures embedded in the inner and outer bacterial membranes and export proteins through a central channel using a related secretion ATPase. The T3SS is thought to have evolved from an ancestral flagellar secretion apparatus by repurposing the molecular machinery for effector delivery rather than motility appendage construction. This homology has implications for understanding pathogen evolution and for designing T3SS inhibitors.

Question 4

T3SS gene expression and needle assembly are constitutively active in pathogenic bacteria throughout their lifecycle.

• T. True
• F. False

Answer: False

T3SS expression is tightly regulated and activated only in response to environmental cues that signal proximity to a host — temperature, pH, ion concentrations, or direct contact with a eukaryotic membrane. This contact-dependent activation prevents the bacterium from wasting energy assembling needle complexes and secreting effectors prematurely, before the pathogen is in a position to infect. The system is loaded but only fires when triggered.

Question 5

How does the T3SS enable bacteria to manipulate host cells from the inside, and why is this more effective than secreting toxins into the extracellular environment?

Model answer: The T3SS injects effector proteins directly into the host cell cytoplasm through a needle that spans both bacterial membranes and punctures the host membrane. Inside the cell, effectors directly bind and repurpose the cell's own signaling machinery — reorganizing the actin cytoskeleton, preventing vacuole-lysosome fusion, suppressing inflammatory signaling. This is far more effective than extracellular toxins because the effectors bypass all extracellular host defenses (antibodies, complement, enzymatic degradation), act on the molecular machinery the pathogen needs to control, and enable precise, localized manipulation that cannot be achieved by molecules that must bind from outside.

The contrast with exotoxins is instructive: a secreted toxin diffuses in the extracellular space and must either find a surface receptor or non-specifically damage cells. T3SS effectors are targeted, intracellular, and strategically positioned to subvert exactly the cellular processes — phagocytosis, lysosomal killing, inflammatory signaling — that would otherwise destroy the pathogen.