Secretion of virulence factors by Escherichia coli

In order to interact with their host, pathogenic strains of E. coli need to secrete some virulence factors which can modify the metabolism of host cel ls, contributing to disease. Since E. coli is a Gram-negative bacteria, thi s secretion process involves the crossing of both the inner and the outer m embranes. E. coli uses mainly four secretion mechanisms called type I, type II, type III and type IV secretion systems. In the type I secretion system , the secretion machinery is composed of three proteins forming a channel t hrough the inner and outer membranes. It is a one-step mechanism. The secre tion signal is present in the carboxyterminal region of the secreted protei n but without proteolytic cleavage. In E. coli, the best studied type I sec reted protein is haemolysin. In type II and type IV secretion systems, the crossing of the inner membrane involves the sec machinery with the cleavage of an aminoterminal signal sequence. The crossing of the outer membrane in volves the formation of a pore either by other proteins (type II) or by the carboxyterminal region of the protein (type IV). The A-B toxins, such as h eat labile enterotoxin, are secreted by the first mechanism and members of the IgA proteases are secreted by the second. The type III secretion system involves at least 20 proteins including cytoplasmic, inner membrane and ou ter membrane proteins. The originality of this system is the ability to inj ect secreted bacteria into the cytosol of the host cells. Such a system is found in attaching and effacing E. coli and in diffusely adhering E, coli. (C) Inra/Elsevier, Paris.