The type III secretion system of Vibrio alginolyticus induces rapid apoptosis, cell rounding and osmotic lysis of fish cells

Abstract

Vibrio alginolyticus is a Gram-negative bacterium and has been recognized as an opportunistic pathogen in humans as well as marine animals. However, the virulence mechanisms for this species of Vibrio have not been elucidated. This study characterized multiple mechanisms that induce cell death in fish cells upon infection with a V. alginolyticus strain, ZJO. The bacterium required its type III secretion system (T3SS) to cause rapid death of infected fish cells. Dying cells exhibited some features of apoptotic cells, such as membrane blebbing, nuclear condensation and DNA fragmentation. Further studies showed that caspase-3 was activated by the T3SS of the ZJO strain, confirming that infection with V. alginolyticus rapidly induces T3SS-dependent apoptosis in fish cells. Infection with the ZJO strain also led to membrane pore formation and release of cellular contents from infected fish cells, as evidenced by lactate dehydrogenase release and the uptake of a membrane-impermeable dye. Importantly, inhibition of apoptosis did not prevent ZJO-infected cells from releasing cellular contents and did not block cell rounding. Taken together, these data demonstrate that infection with V. alginolyticus may promote at least three different T3SS-dependent events, which lead to the death of fish cells. This study provides an important insight into the mechanism used by Vibrio species to cause host-cell death.