Multilocus sequence analysis of diverse Streptococcus iniae isolates indicates an underlying genetic basis for phenotypic heterogeneity

Authors: HECKMAN, TAYLOR - University Of California; GRIFFIN, MATT - Mississippi State University; CAMUS, ALVIN - University Of Georgia; Lafrentz, Benjamin; MORICK, DANNY - University Of Haifa; SMIRNOV, RITA - Ministry Of Agriculture - Israel; OFEK, TAMIR - Ministry Of Agriculture - Israel; SOTO, ESTEBAN - University Of California

Submitted to: Diseases of Aquatic Organisms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2020
Publication Date: 9/17/2020
Citation: Heckman, T.I., Griffin, M.J., Camus, A.C., Lafrentz, B.R., Morick, D., Smirnov, R., Ofek, T., Soto, E. 2020. Multilocus sequence analysis of diverse Streptococcus iniae isolates indicates an underlying genetic basis for phenotypic heterogeneity. Diseases of Aquatic Organisms. 141:53-69. https://doi.org/10.3354/dao03521.
DOI: https://doi.org/10.3354/dao03521

Interpretive Summary: Streptococcus iniae is an economically important Gram-positive bacterial pathogen of aquaculture raised fish species and accounts for millions of dollars of losses annually worldwide. The genetic diversity of this important fish pathogen is not fully understood, and it has been suggested that this lack of knowledge has made development of vaccines a challenge. This study genetically and phenotypically characterized eleven S. iniae isolates from diseased wild and farmed fish from North America, Central America and the Caribbean. High resolution genetic methods were developed and used to compare these isolates to 84 other strains of Streptococcus spp. relevant to aquaculture. The results separated the isolates into distinct groups with biological relevance in terms of where the bacteria were isolated and what fish species they were isolated from. Various laboratory experiments were conducted to determine the virulence of the isolates and the results demonstrated differences with some correlation to the distinct groups identified using genetic methods. This research increases our knowledge of the genetic diversity of S. iniae and is important for developing practical solutions to combat this important fish pathogen.

Technical Abstract: Streptococcus iniae is a Gram-positive, opportunistically zoonotic bacterium infective to a wide variety of farmed and wild fish species worldwide. Outbreaks in wild fish can have detrimental environmental and cultural impacts, and mortality events in aquaculture can result in significant economic losses. As an emerging or re-emerging pathogen of global significance, understanding the coalescing factors that contribute to piscine streptococcosis is crucial for developing strategies to control infections. Intraspecific antigenic and genetic variability of S. iniae has made development of autogenous vaccines a challenge, particularly where the diversity of locally endemic S. iniae strains is unknown. This study genetically and phenotypically characterized 11 S. iniae isolates from diseased wild and farmed fish from North America, Central America and the Caribbean. A multilocus sequence analysis (MLSA) scheme was developed to phylogenetically compare these isolates to 84 other strains of Streptococcus spp. relevant to aquaculture. MLSA generated phylogenies comparable to established genotyping methods, and isolates formed distinct clades related to geography and host type. The endothelial Oreochromis mossambicus bulbus arteriosus cell line and whole blood from rainbow trout (Oncorhynchus mykiss), Nile tilapia (Oreochromis niloticus), and white sturgeon (Acipenser transmontanus) were used to investigate the persistence and virulence of the 11 piscine isolates using in vitro assays. In vivo challenges using an O. niloticus challenge model were used to evaluate virulence by the intra-gastric route of infection. Isolates showed significant differences (p<0.05) in virulence and persistence, with some correlation to genogroup, establishing a basis for further work uncovering genetic factors leading to increased pathogenicity.