Helicobacter pylori infection in humans is associated with diverse of clinical outcomes which are partly attributed to bacterial strain differences. Secreted bacterial products are thought to be involved in the pathogenesis caused by this non-invasive bacterium. Electron microscopy of gastric biopsies from infected individuals revealed blebbing of the H. pylori outer membrane, similar to the process of outer membrane vesicle shedding which occurs when the bacterium is grown in broth. Porins, a class of proinflammatory proteins, were observed in the outer membrane vesicles. The VacA cytotoxin, which is produced by 50-60% of H. pylori strains and associated with increased pathogenesis of infection, was also found to be vesicle-associated and biologically active. This supports the hypothesis that these vesicles represent a vehicle for the delivery of damaging bacterial products to the gastric mucosa.