Innate Immune Response to Streptococcus iniae Infection in Zebrafish Larvae

Abstract

Streptococcus iniaecauses systemic infection characterized by meningitis and sepsis. Here, we report a larval zebrafish model ofS. iniaeinfection. Injection of wild-typeS. iniaeinto the otic vesicle induced a lethal infection by 24 h postinfection. In contrast, anS. iniaemutant deficient in polysaccharide capsule (cpsAmutant) was not lethal, with greater than 90% survival at 24 h postinfection. Live imaging demonstrated that both neutrophils and macrophages were recruited to localized otic infection with mutant and wild-typeS. iniaeand were able to phagocytose bacteria. Depletion of neutrophils and macrophages impaired host survival following infection with wild-typeS. iniaeand thecpsAmutant, suggesting that leukocytes are critical for host survival in the presence of both the wild-type and mutant bacteria. However, zebrafish larvae with impaired neutrophil function but normal macrophage function had increased susceptibility to wild-type bacteria but not thecpsAmutant. Taking these findings together, we have developed a larval zebrafish model ofS. iniaeinfection and have found that although neutrophils are important for controlling infection with wild-typeS. iniae, neutrophils are not necessary for host defense against thecpsAmutant.