Comparative genomics among members of the Streptococcus bovis/Streptococcus equinus complex

Background: Today, only one streptococcal species, i.e. Streptococcus thermophilus is recognized as food-grade. Interestingly, other streptococci like Streptococcus macedonicus and Streptococcus infantarius belonging to the Streptococcus bovis/Streptococcus equinus complex (SBSEC) are also found in food matrices. However, these species are phylogenetically related to Streptococcus gallolyticus and Streptococcus pasteurianus that have been linked to endocarditis, bacteremia and colon cancer. Objectives: To compare the available genomes of the members of the SBSEC in order to shed light onto their evolution and phylogenetic relation and to assess in silico their pathogenic potential. Methods: Comparative genomics analysis including full chromosome and CDS alignments, whole genome phylogeny and evaluation of gene content (e.g. core genome, singletons, etc.) was performed with appropriate bioinformatics tools. Conclusions: Despite the fact that the four species of the SBSEC were found tightly related based on whole genome phylogeny, there were two different patterns of evolution among them. Streptococcus pasteurianus, S. macedonicus and S. infantarius seem to have undergone a reductive evolution process that resulted in significantly diminished genome sizes and increased percentages of potential pseudogenes when compared to S. gallolyticus. In addition, S. pasteurianus, S. macedonicus and S. infantarius seem to have lost several genes previously linked to the ability of S. gallolyticus to survive in the gastrointestinal tract of herbivores and to its pathogenicity. Our findings indicate differences in the ecological niche and the pathogenic potential among the four species