Abstract
BackgroundStreptococcus mutans is the major pathogen of dental caries, and it occasionally causes infective endocarditis. While the pathogenicity of this species is distinct from other human pathogenic streptococci, the species-specific evolution of the genus Streptococcus and its genomic diversity are poorly understood.ResultsWe have sequenced the complete genome of S. mutans serotype c strain NN2025, and compared it with the genome of UA159. The NN2025 genome is composed of 2,013,587 bp, and the two strains show highly conserved core-genome. However, comparison of the two S. mutans strains showed a large genomic inversion across the replication axis producing an X-shaped symmetrical DNA dot plot. This phenomenon was also observed between other streptococcal species, indicating that streptococcal genetic rearrangements across the replication axis play an important role in Streptococcus genetic shuffling. We further confirmed the genomic diversity among 95 clinical isolates using long-PCR analysis. Genomic diversity in S. mutans appears to occur frequently between insertion sequence (IS) elements and transposons, and these diversity regions consist of restriction/modification systems, antimicrobial peptide synthesis systems, and transporters. S. mutans may preferentially reject the phage infection by clustered regularly interspaced short palindromic repeats (CRISPRs). In particular, the CRISPR-2 region, which is highly divergent between strains, in NN2025 has long repeated spacer sequences corresponding to the streptococcal phage genome.ConclusionThese observations suggest that S. mutans strains evolve through chromosomal shuffling and that phage infection is not needed for gene acquisition. In contrast, S. pyogenes tolerates phage infection for acquisition of virulence determinants for niche adaptation.
Highlights
Streptococcus mutans is the major pathogen of dental caries, and it occasionally causes infective endocarditis
The chromosomal inversion across the replication axis between S. mutans strains occurred between homologous ribosomal operons located almost the same distance from the ori region as the S. pyogenes chromosomal inversion
This chromosomal inversion, a characteristic X-shaped symmetrical DNA dot plot, is found between S. mutans and S. pyogenes strains and between S. mutans strains and all the streptococcal species in which the complete genome sequence is known. These observations indicate that the chromosomal inversion across the replication axis has occurred frequently within streptococci during evolution
Summary
Streptococcus mutans is the major pathogen of dental caries, and it occasionally causes infective endocarditis. Even though organisms belonging to the same genus/ species have a common gene set (the core genome), individual organisms differ (strain-specific genes) in ways representative of the physiological and virulence properties of an organism [2,3]. Not all genetic differences between strains are important for niche adaptation of the bacteria, strain-specific genes are thought to be responsible for the survival of an organism in its chosen niche This variation can be due to genetic noise (i.e., indels, mobile- and selfish DNA) [4,5], gene loss [6,7], gene duplication [8] or modification of some of the existing genes [9,10]. S. pyogenes belongs to the same genus, it has a smaller pan-genome and greater level of recombination in its core genome [12] These organisms provide a good model for identifying the causes of genome plasticity in human pathogens
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call