type III secretion system
Summary
The type III secretion system is a needle-like molecular syringe used by gram-negative bacteria to inject effector proteins directly from their cytoplasm into a host cell. It is often called the injectisome and is a key virulence determinant.
Detail
Structurally related to the flagellar basal body, the apparatus spans both bacterial membranes and the host membrane, delivering effectors without exposing them to the extracellular environment or to antibody. Its users include Yersinia, which injects Yop proteins to disable phagocytes; Salmonella, whose effectors trigger membrane ruffling and bacterial uptake by M cells of Peyer patches; Shigella, which escapes the phagosome and spreads cell to cell by actin polymerization; enteropathogenic and enterohaemorrhagic E. coli, which inject Tir to create the attaching-and-effacing lesion that flattens microvilli; and Pseudomonas aeruginosa, whose ExoU and ExoS contribute to acute lung injury. Because the system is essential for virulence but not for viability, it is an attractive target for anti-virulence therapeutics that would exert less selective pressure for resistance than conventional antibiotics. It should be distinguished from type IV secretion, which transfers DNA in conjugation and is used by Helicobacter pylori to deliver CagA, and from type II secretion, which exports folded proteins such as cholera toxin.
Sources
- Levinson Medical Microbiology and Immunology
- Molecular Biology of the Cell
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