UC Davis

Streptococcus iniae is a gram-positive bacterium and one of the primary etiologic agents of piscine streptococcosis, a pervasive disease that costs the global aquaculture industry billions of dollars in annual losses. Treatment by antimicrobial administration and prevention by vaccination have had limited success in controlling S. iniae. The primary obstacle for developing effective and sustainable control strategies is a lack of understanding regarding the genetic and antigenic diversity of S. iniae in relation to its pathogenesis. As a re-emerging pathogen, our understanding of the geographic and host range of S. iniae is still expanding. Since the first isolation of S. iniae from skin lesions of a captive Amazon river dolphin (Inia geoffrensis) in 1976, it has been reported from almost 100 wild, farmed, and ornamental fish species inhabiting fresh, euryhaline and marine environments across every continent except Antarctica. Additionally, S. iniae is an opportunistic zoonotic pathogen, capable of causing systemic disease in humans and other mammals. The work presented in this dissertation aims to elucidate the genetics, ecology, and infection dynamics of strains from varied isolation sources with translatable results for aquaculture. A multilocus sequence analysis (MLSA) scheme was developed to understand the genetic relationships between diverse isolates and uncover shared phenotypic and virulence characteristics. The MLSA phylogenies were comparable to established genotyping methods and placed the S. iniae isolates into 5 major clades relating to phenotype and host species. Characterized strains from different genetic backgrounds were assessed for their ability to form biofilms, and for the role of biofilms in environmental persistence and resistance to treatment and disinfection. All strains formed biofilms within 72 hours using the minimum biofilm eradication concentration (MBEC) assay® system, and biofilms drastically increased the aquatic persistence of S. iniae and resistance to antimicrobials commonly used in aquaculture. Finally, live-attenuated vaccine candidates were developed from representative strains from North American clades by rifampin passaging. Candidate strains were attenuated in virulence by in vitro and in vivo assays and elicited a protective immune response in tilapia following intra-coelomic immunization.