SummaryThe α α α α -haemolysin is an important virulence factor commonly expressed by extraintestinal pathogenic Escherichia coli . The secretion of the α α α 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 α α α -haemolysin appeared very tightly associated with the OMVs as judged by dissociation assays and proteinase susceptibility tests. The α α α α -haemolysin in OMVs was cytotoxically active and caused lysis of red blood cells. The OMVs containing the α α α α -haemolysin were distinct from the OMVs not containing α α α 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 α α 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 α α α -haemolysin from the bacteria to the host cells during bacterial infections.
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