Cpa, the Outer Membrane Protease of Cronobacter sakazakii, Activates Plasminogen and Mediates Resistance to Serum Bactericidal Activity

ABSTRACTCronobacterspp. are emerging neonatal pathogens in humans, associated with outbreaks of meningitis and sepsis. To cause disease, they must survive in blood and invade the central nervous system by penetrating the blood-brain barrier. C. sakazakiiBAA-894 possesses an ∼131-kb plasmid (pESA3) that encodes an outer membrane protease (Cpa) that has significant identity to proteins that belong to the Pla subfamily of omptins. Members of this subfamily of proteins degrade a number of serum proteins, including circulating complement, providing protection from the complement-dependent serum killing. Moreover, proteins of the Pla subfamily can cause uncontrolled plasmin activity by converting plasminogen to plasmin and inactivating the plasmin inhibitor α2-antiplasmin (α2-AP). These reactions enhance the spread and invasion of bacteria in the host. In this study, we found that an isogenic cpamutant showed reduced resistance to serum in comparison to its parent C. sakazakiiBAA-894 strain. Overexpression of Cpa in C. sakazakiior Escherichia coliDH5α showed that Cpa proteolytically cleaved complement components C3, C3a, and C4b. Furthermore, a strain of C. sakazakiioverexpressing Cpa caused a rapid activation of plasminogen and inactivation of α2-AP. These results strongly suggest that Cpa may be an important virulence factor involved in serum resistance, as well as in the spread and invasion of C. sakazakii.