Dynamic interaction of neutrophils and RFP-labelled Vibrio parahaemolyticus in zebrafish (Danio rerio)

Vibrio parahaemolyticus (Vp) is an aquatic zoonotic pathogen that causes vibriosis in marine animals as well as sepsis, gastroenteritis and wound infection in human. In vertebrates, the innate immune system plays a critical defense mechanism against Vp infection including transmigration of neutrophils. In this study, we have examined the genetic recombination and infectious process of Vp in the zebrafish (Danio rerio), a vertebrate model system extensively used for studying host-pathogen interactions. A pathogenic Vp strain, Vp57, tagged with red fluorescent protein (RFP) (Vp57RFP) was introduced into larval zebrafish at 3 days post fertilization (dpf), in which the innate immunity is present whereas the adaptive immunity has not yet developed. Vp57 and Vp57RFP showed similar LD50 and induced similar symptoms and pathological changes in the hosts. We microinjected 579 colony-forming units (CFU) Vp57RFP into the zebrafish caudal vein, and observed that neutrophils were recruited to the injection site and within 3 h post infection (hpi) Vp57RFP were mainly distributed in the tail, eyes, heart and optic vesicle. After 3 hpi, the fish died with slight spine bending. Several Vp57RFP were also detected in somites, the phagocytosis of neutrophils was activated through the progress of bacterial infections. We found that chemotaxis and phagocytosis of neutrophils occurs when zebrafish is infected by Vp57RFP, whereas Vp57RFP can escape from neutrophils and colonize other remote regions using the blood circulation system.