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Diseases of Aquatic Organisms

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DAO 141:53-69 (2020)  -  DOI:

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

Taylor I. Heckman1, Matt J. Griffin2, Alvin C. Camus3, Benjamin R. LaFrentz4, Danny Morick5, Rita Smirnov6, Tamir Ofek6, Esteban Soto1,*

1Aquatic Animal Health Laboratory, Department of Medicine & Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA
2Department of Pathobiology and Population Medicine, College of Veterinary Medicine, Mississippi State University, Stoneville, MS 39762, USA
3College of Veterinary Medicine, University of Georgia, Athens, GA 30602, USA
4USDA-ARS, Aquatic Animal Health Research Unit, Auburn, AL 36832, USA
5Morris Kahn Marine Research Station, Department of Marine Biology, Leon H. Charney School of Marine Sciences, University of Haifa, Haifa 3498838, Israel
6Central Fish Health Laboratory, Department of Fisheries and Aquaculture, Ministry of Agriculture & Rural Development, Nir David 1080300, Israel
*Corresponding author:

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 contributing 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 phenotype and host species. 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 isolates using in vitro assays. In vivo challenges using an O. niloticus model were used to evaluate virulence by the intragastric 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.

KEY WORDS: Streptococcus iniae · Multilocus sequence analysis · MLSA · Streptococcosis · Aquaculture · Genotype

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Cite this article as: Heckman TI, Griffin MJ, Camus AC, LaFrentz BR and others (2020) Multilocus sequence analysis of diverse Streptococcus iniae isolates indicates an underlying genetic basis for phenotypic heterogeneity. Dis Aquat Org 141:53-69.

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