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Whole genome sequencing reveals possible host species adaptation of Streptococcus dysgalactiae

분류 국외뉴스 > 과학 등록일 2021-09-02
출처 nature 조회 62
첨부파일
내용바로가기 https://www.nature.com/articles/s41598-021-96710-z

 

Whole genome sequencing reveals possible host species adaptation of Streptococcusdysgalactiae
 
Abstract
 
Streptococcus dysgalactiae (SD) is an emerging pathogen in human and veterinary medicine, and is associated with several host species, disease phenotypes and virulence mechanisms. SD has traditionally been divided into the subspecies dysgalactiae (SDSD) and subsp. equisimilis (SDSE), but recent molecular studies have indicated that the phylogenetic relationships are more complex. Moreover, the genetic basis for the niche versatility of SD has not been extensively investigated. To expand the knowledge about virulence factors, phylogenetic relationships and host-adaptation strategies of SD, we analyzed 78 SDSD genomes from cows and sheep, and 78 SDSE genomes from other host species. Sixty SDSD and 40 SDSE genomes were newly sequenced in this study. Phylogenetic analysis supported SDSD as a distinct taxonomic entity, presenting a mean value of the average nucleotide identity of 99%. Bovine and ovine associated SDSD isolates clustered separately on pangenome analysis, but no single gene or genetic region was uniquely associated with host species. In contrast, SDSE isolates were more heterogenous and could be delineated in accordance with host. Although phylogenetic clustering suggestive of cross species transmission was observed, we predominantly detected a host restricted distribution of the SD-lineages. Furthermore, lineage specific virulence factors were detected, several of them located in proximity to hotspots for integration of mobile genetic elements. Our study indicates that SD has evolved to adapt to several different host species and infers a potential role of horizontal genetic transfer in niche specialization.
 
Introduction
 
Streptococcus dysgalactiae (SD) is a potent pathogen capable of producing a wide spectrum of clinical manifestations and infecting a broad range of host species. Based on DNA-relatedness and phenotypic characteristics, SD is divided into Streptococcus dysgalactiae subspecies dysgalactiae (SDSD) and subspecies equisimilis (SDSE)1. SDSD are alpha-haemolytic or non-haemolytic strains belonging to Lancefield group C that are mainly associated with animals, while SDSE are beta-haemolytic strains belonging to Lancefield groups A,C, G or L, and cause miscellaneous infections in humans and domestic animals1.
 
SDSD is reported as an important pathogen in meat sheep and dairy cows. The pathogenesis in the two host species is remarkably different. In sheep flocks, SDSD-infections are associated with outbreaks of septic arthritis in lambs less than four weeks old, whilst in bovine dairy herds SDSD is a frequent cause of mastitis.
 
In Norway, the relative importance of SDSD-infections in livestock has increased over the last decade. In a survey from 2018, 5.6% of 1700 Norwegian sheep farms had experienced outbreaks of infectious arthritis in lambs2. At the same time, the prevalence of SDSD intramammary infections is increasing in bovine dairy herds, and SDSD is now the third most common cause of clinical mastitis in dairy cows in Norway3. The two industries currently define streptococcal mastitis in dairy cows and streptococcal joint infections in lambs to be among the major challenges in Norwegian livestock production, because of their negative effects on animal health and welfare, production and antibiotic usage.
 
In the past decades, SDSE has emerged as an important human pathogen. Traditionally, SDSE in humans has been regarded as a potentially zoonotic pathogen, but recent phylogenetic studies based on multilocus sequence typing (MLST) have suggested distinct host-adapted subpopulations of SDSE4,5.
 
Although several studies have involved whole genome sequencing of SDSE, very few studies have sequenced SDSD6,7. Genomic investigations of SDSD to reveal factors associated with virulence, persistence in the environment and host specificity can contribute to enhancing our understanding of pathogenicity and transmission. The purpose of this study was to explore by whole genome sequencing the diversity of SDSD from sheep and cattle in Norway, and to identify genetic factors that might contribute to host adaptation. Furthermore, a comparative genome analysis was performed between bovine- and ovine associated SDSD, and SDSE from various other host species.