Many bacterial species have evolved various types of secretion system. Type VI secretion systems are naturally found in gram negative organisms, including Serratia species, Vibrio cholerae and Pseudomonas aeruginosa. A type VI secretion system has also been found in Salmonella typhimurium, which is closely related to Escherichia coli (E. coli). The proteins for Salmonella type VI secretion systems are encoded by more than 13 genes, including Hcp, which encodes for the main structural component of the needle. This projects through the periplasm and outer membrane and can inject directly into competing cells, via the tip protein which is encoded by the gene VgrG. In this way, the type VI secretion system punctures other cells. Hcp and VgrG are largely conserved across all species expressing these systems. TypeVi secretion systems can also be associated with secreted effector molecules, these are thought to play a role in the pathogenesis of higher organisms and could help facilitate interactions with other bacteria.
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