Streptococcus salivarius is a commensal bacterium that normally inhabits the oral mucosa. However, preliminary data indicated that the strain... Show moreStreptococcus salivarius is a commensal bacterium that normally inhabits the oral mucosa. However, preliminary data indicated that the strain ATCC 25975 has acquired parasitic genes from Streptococcus pneumoniae. To investigate the nature of this genetic exchange, the genome of Streptococcus salivarius ATCC 27945 was first sequenced and served as a comparative model to provide insight into the possibility of development of pathogenicity within the salivarius group. Illumina and PacBio sequencing data were used complementarily to generate reliable genomes of the Streptococcus salivarius ATCC 27945 and ATCC 25975 strains. The reads were trimmed, filtered, assembled and annotated using custom Perl scripts and various software. The completed genomes of S. salivarius ATCC 27945 and ATCC 25975 are 2.11 Mbp and 2.20 Mbp long, respectively, with ATCC 25975 featuring an additional plasmid. Comparative genomics with other sequenced salivarius genomes revealed that strain ATCC 27945 was most closely related to strains JIM8777 and NCTC8618, while strain ATCC 25975 was more closely related to strains 57I and CCHSS3. The proteins that were common across the investigated salivarius genomes included housekeeping proteins involved in pathways such as DNA replication, metabolism and DNA repair but the unique protein types and their relative location to each other within the ATCC main chromosomes did not provide conclusive evidence to the identification of the parasitic cassette. However, the plasmid contained in S. salivarius ATCC 25975 showed promising signs of containing these genes of interest. Several genes typically found in S. pneumoniae such as capsular polysaccharide genes and two copies of integrative and conjugative genes were identified in close proximity. The phylogenetic analysis of the two S. salivarius ATCC strains suggested that the proposed genes acquired from S. pneumoniae might have been gained via horizontal gene transfer rather than through sporadic mutations. The plasmid had a 5.1% smaller GC content compared to the other salivarius chromosomes, further suggesting that it was acquired from a distinct organism. Overall, while these results provided useful insights into the nature and location of the parasitic cassette, further investigations will be required to assess the full extent of the genetic shuffling that occurred by horizontal gene transfer in these Streptococcus species. M.S. in Biology, July 2015 Show less
