Microevolution of aquaticStreptococcus agalactiaeST-261 from Australia indicates dissemination via imported tilapia and ongoing adaptation to marine hosts or environment

Abstract

Streptococcus agalactiae (GBS) causes disease in a wide range of animals. The serotype 1b lineage is highly adapted to aquatic hosts, exhibiting substantial genome reduction compared with terrestrial con-specifics. Here we sequence genomes from 40 GBS isolates including 25 from wild fish and captive stingrays in Australia, six local veterinary or human clinical isolates, and nine isolates from farmed tilapia in Honduras and compare with 42 genomes from public databases. Phylogenetic analysis based on non-recombinant core genome SNPs indicated that aquatic serotype Ib isolates from Queensland were distantly related to local veterinary and human clinical isolates. In contrast, Australian aquatic isolates are most closely related to a tilapia isolate from Israel, differing by only 63 core-genome SNPs. A consensus minimum spanning tree based on core genome SNPs indicates dissemination of ST-261 from an ancestral tilapia strain, which is congruent with several introductions of tilapia into Australia from Israel during the 1970s and 1980s. Pan-genome analysis identified 1,440 genes as core with the majority being dispensable or strain-specific with non-protein-coding intergenic regions (IGRs) divided amongst core and strain-specific genes. Aquatic serotype Ib strains have lost many virulence factors during adaptation, but six adhesins were well conserved across the aquatic isolates and might be critical for virulence in fish and targets for vaccine development. The close relationship amongst recent ST-261 isolates from Ghana, USA and China with the Israeli tilapia isolate from 1988 implicates the global trade in tilapia seed for aquaculture in the widespread dissemination of serotype Ib fish-adapted GBS.ImportanceStreptococcus agalactiae (GBS) is a significant pathogen of humans and animals. Some lineages have become adapted to particular hosts and serotype Ib is highly specialized to fish. Here we show that this lineage is likely to have been distributed widely by the global trade in tilapia for aquaculture, with probable introduction into Australia in the 1970s and subsequent dissemination in wild fish populations. We report variability in the polysaccharide capsule amongst this lineage, but identify a cohort common surface proteins that may be a focus of future vaccine development to reduce the biosecurity risk in international fish trade.

Evolutionary epidemiology ofStreptococcus iniae: linking mutation rate dynamics with adaptation to novel immunological landscapes

“…Hewitt broth with the DNeasy Blood & Tissue kit (Qiagen) using a modified protocol with an additional 463 lysis step as described previously (Kawasaki, et al 2018). gDNA was analysed on agarose gel, quantified 464 by Qubit fluorimetry (Invitrogen), and 16S rRNA gene sequenced using universal 27F and 1492R primers 465 (Amann, et al 1995), to confirm integrity, identity, and purity.…”

A pan genome reverse vaccinology approach to prevent Streptococcus agalactiae infection in farmed tilapia

“…Since three ST-261 adhesins were identified across all GBS strains (Kawasaki et al, 2018), recombinant ST-261 adhesin 0337, 0626 and 1196 were produced expressed in E. coli.…”

“…3.6A-D). six novel surface proteins, including three putative adhesins specific to serotype Ib piscine strains, one is specific to ST-261 isolate, and three adhesins identified in all GBS strains in the pan-genome (Kawasaki et al, 2018). Due to their critical roles in early stages of infection, adhesins are considered as good vaccine candidate antigens (Nobbs et al, 2009;Raynes et al, 2018).…”

“…We sequenced the genomes of GBS isolates from wild and captive fish, along with collocated terrestrial farm, domestic animal and human clinical isolates in Australia and assembled a pan-genome based on these strains along with complete genome sequences of GBS available in the NCBI database (Kawasaki et al, 2018). The pan-genome analysis revealed that, although serotype Ib piscine strains have lost approximately 60% of their virulence factor repertoire compared with terrestrial conspecific and serotype Ia piscine strains, a number of virulence factors including hyaluronate lyase (exoenzyme), CPS and six surface protein, which were identified as adhesins, are conserved (Kawasaki et al, 2018).…”

“…Margarit et al (2009) found that at least one of the three pilus islands were conserved among the all clinical isolates used in their study. Previously, we have shown that all GBS sequence type (ST) 261 strains carry the six adhesins (ST-261 adhesins), but only three of them are conserved within all strains including terrestrial isolates, while one is exclusive to ST-261 and two are specific to serotype Ib piscine strains (Kawasaki et al, 2018). Therefore, in this study, we focus on four ST-261 adhesins, three which are conserved in all strains and one which is specific to serotype Ib fish strains.…”