Abstract
Background Streptococcus suis is a zoonotic pathogen that infects pigs and can occasionally cause serious infections in humans. S. suis infections occur sporadically in human Europe and North America, but a recent major outbreak has been described in China with high levels of mortality. The mechanisms of S. suis pathogenesis in humans and pigs are poorly understood.Methodology/Principal FindingsThe sequencing of whole genomes of S. suis isolates provides opportunities to investigate the genetic basis of infection. Here we describe whole genome sequences of three S. suis strains from the same lineage: one from European pigs, and two from human cases from China and Vietnam. Comparative genomic analysis was used to investigate the variability of these strains. S. suis is phylogenetically distinct from other Streptococcus species for which genome sequences are currently available. Accordingly, ∼40% of the ∼2 Mb genome is unique in comparison to other Streptococcus species. Finer genomic comparisons within the species showed a high level of sequence conservation; virtually all of the genome is common to the S. suis strains. The only exceptions are three ∼90 kb regions, present in the two isolates from humans, composed of integrative conjugative elements and transposons. Carried in these regions are coding sequences associated with drug resistance. In addition, small-scale sequence variation has generated pseudogenes in putative virulence and colonization factors.Conclusions/SignificanceThe genomic inventories of genetically related S. suis strains, isolated from distinct hosts and diseases, exhibit high levels of conservation. However, the genomes provide evidence that horizontal gene transfer has contributed to the evolution of drug resistance.
Highlights
Streptococcus suis is a Gram positive coccus that colonises pigs
The genome of S. suis strain P1/7, a pig disease isolate The genome of Streptococcus suis P1/7 consists of a single circular chromosome of 2,007,491 bp (Figure 1; accession number AM946016) containing 1,908 predicted protein coding sequences (CDSs; Table 1), 82 of which are pseudogenes or gene fragments (Table S1)
Some functional groups such as cofactor biosynthesis and amino acid biosynthesis exhibit a lower level of conservation and probably reflect the available nutrients in the niches occupied by the individual streptococcal species
Summary
Streptococcus suis is a Gram positive coccus that colonises pigs. While it is generally carried asymptomatically in adult pigs, it can cause severe systemic disease in piglets, manifested as a rapidly fatal sepsis associated with meningitis, polyarthritis and pneumonia. Why adult pigs carry the causative bacteria asymptomatically while piglets develop acute disease is unknown. The main site of carriage in the adult are the tonsils, but bacteria have been isolated from the nasal cavities, the gastrointestinal tract and genital tract. S. suis has been isolated from a range of other mammalian and avian species [1]. Streptococcus suis is a zoonotic pathogen that infects pigs and can occasionally cause serious infections in humans. The mechanisms of S. suis pathogenesis in humans and pigs are poorly understood
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