Type III (contact-dependent) secretion in Gram-negative bacteria

Contact-dependent or type III secretion systems are used by bacterial pathogens, on host cell contact, to deliver bacterial virulence proteins directly into the host cell cytoplasm. Type III secretion systems have been shown to be crucial for virulence in various species. The secretion machinery comprises at least 17 proteins which span the cell envelope and are highly conserved between the various bacterial species in which the system occurs: it is present only in Gramnegative plant and animal pathogens, including species of the genera Yersinia, Salmonella, Shigella, Pseudomonas, Escherichia, Bordetella, Chlamydia, Porphyromonas, Erwinia, Xanthomonas and Rhizobium. The purpose of the system differs from one species to another reflecting the lifestyle of each pathogen, due to the secretion of different effector proteins. As the proteins of bacterial type III secretion system and the flagellar assembly have significant homology and similarities of function, and because the bacterial flagellum appears to have existed long before Gram-negative bacteria, it has been proposed that the flagellar assembly may be the ancestor of the type III system. (C) 1999 Lippincott Williams & Wilkins.