Genome sequencing and comparative genome analysis of the emerging fish pathogen Streptococcus dysgalactiae subsp. Dysgalactiae

Abstract

Streptococcus dysgalatiae subsp. dysgalactiae (SDD) is a Gram-positive cocci, that autoaggregates in saline solution, it is catalase negative and forms long chains in growth medium. On fish, the disease is characterized with clinical signs of septicaemia and a typical form of necrosis in the caudal peduncle with a high mortality rate. In 2002, it caused the first outbreak in southern Japanese farms and during the subsequent years fish farms all over the country suffered losses. On Brazil, outbreaks of streptococcosis are common in the freshwaterfish species Nile tilapia, Oreochromis niloticus (L.) and in 2007, the first disease outbreak caused by SDD was spotted on the state of Ceará. Nowadays it is considered as an emergent pathogen therefore, considering the importance of a complete genome to characterize thispathogen; a next-generation sequence genome initiative was managed. Three strains, SD64, SD92 and SD192, were sequenced and assembled in order to perform genomic comparative analysis within other SD strains. Results confirm robust and coherent cluster within S. dysgalactiae subsp. equisimilis (SDE) and SDD strains. MLST analysis also showed additional host dependent clustering within SDD strains, this presumably shows that the SDD strains maybe host-adapted. Plus, higher similarity within SDE strains than between SDD strains reveals that even within the same subespecies the strains have different features among them.Final results propose SDD adaptation to changing environments and new hosts presumably involved with the acquisition of virulence factor and other features from other species