Release of the type I secreted α-haemolysin via outer membrane vesicles from Escherichia coli

Abstract

The α-haemolysin is an important virulence factor commonly expressed by extraintestinal pathogenic Escherichia coli. The secretion of the α-haemolysin is mediated by the type I secretion system and the toxin reaches the extracellular space without the formation of periplasmic intermediates presumably in a soluble form. Surprisingly, we found that a fraction of this type I secreted protein is located within outer membrane vesicles (OMVs) that are released by the bacteria. The α-haemolysin appeared very tightly associated with the OMVs as judged by dissociation assays and proteinase susceptibility tests. The α-haemolysin in OMVs was cytotoxically active and caused lysis of red blood cells. The OMVs containing the α-haemolysin were distinct from the OMVs not containing α-haemolysin, showing a lower density. Furthermore, they differed in protein composition and one component of the type I secretion system, the TolC protein, was found in the lower density vesicles. Studies of natural isolates of E. coli demonstrated that the localization of α-haemolysin in OMVs is a common feature among haemolytic strains. We propose an alternative pathway for the transport of the type I secreted α-haemolysin from the bacteria to the host cells during bacterial infections. © 2005 Blackwell Publishing Ltd.