Milk adaptation and pathogenic potential among members of the Streptococcus bovis/Streptococcus equinus complex

The Streptococcus bovis/Streptococcus equinus complex (SBSEC) consists of species commonly found in the gastrointestinal tract (GIT) of herbivores. Certain members of the complex like Streptococcus macedonicus and Streptococcus infantarius are frequently isolated from traditional fermented foods, mostly of dairy origin. However, some species of this complex, like Streptococcus gallolyticus and Streptococcus pasteurianus are known pathogens of humans involved in a range of diseases including endocarditis, menengitis, bacteremia, colon cancer etc. Analysis of the genome of S. macedonicus strain ACA-DC 198 isolated from traditional Greek Kasseri cheese revealed important traits of adaptation to the dairy environment. The strain contains a significant percentage of potential pseudogenes indicating that it may have evolved through genome decay processes. Streptococcus macedonicus has an extra gene cluster for lactose and galactose metabolism unique among the SBSEC members and a typical proteolytic system required for casein hydrolysis. Furthermore, we found evidence in the genome of S. macedonicus suggesting HGT (horizontal gene transfer) events with potential donors Lactococcus lactis and Streptococcus thermophilus. Perhaps the most pronounced among these HGT events is the presence in S. macedonicus of pSMA198 which belongs to the lactococcal pCI305/pWV02 family of plasmids. Our analysis suggests that pSMA198 may have been acquired by S. macedonicus from L. lactis. Analogous traits have been reported for S. infantarius suggesting that at least these two strains present adaptations to the dairy environment among SBSEC. Finally, we were able to identify a number of potential virulence factors (VFs) within the SBSEC members whose presence varied among species of the complex. Streptococcus macedonicus and S. infantarius miss some of the VFs present in S. gallolyticus suggesting a diminished pathogenic potential for the two species