Comparative genomics of Streptococcus macedonicus ACA-DC 198 against related species within the Streptococcus bovis/Streptococcus equinus complex

Apart from Streptococcus thermophilus other streptococci that can be found growing in milk belong to the Streptococcus bovis/Streptococcus equinus complex (SBSEC). Interestingly, Streptococcus macedonicus, which is a member of SBSEC, has been suggested to be adapted to milk and to be nonpathogenic. However, the species is phylogenetically related to Streptococcus gallolyticus and Streptococcus pasteurianus (formerly known as S. bovis biotypes I and II.2, respectively), which in turn are considered pathogenic, since they have been implicated in endocarditis and colon cancer in humans. Comparative analysis of the S. macedonicus genome with the complete genomes of its related streptococci (including that of S. infantarius, which is also a dairy isolate) indicated that a significant portion of the genomic organization has been conserved overall. Following a gene presence/absence strategy, we determined that S. macedonicus shows a reduced capacity to reside in the gastrointestinal tract of ruminants when compared to S. gallolyticus since it misses important genes for metabolizing complex carbohydrates of plant origin and for detoxifying this environment. S. macedonicus also lacks several pathogenicity traits found in S. gallolyticus. For example from the three pilus gene clusters (pil1, pil2, pil3), which may mediate the binding of S. gallolyticus to the extracellular matrix, S. macedonicus carries only one (i.e. the pil3). Gene gain events are also evident in the S. macedonicus genome sometimes originating from dairy bacteria, like the acquisition of the lactococcal plasmid pSMA198. Functional analysis of the S. macedonicus genome is necessary to further assess its pathogenic and technological potential