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Background    Streptococcus macedonicus  is an intriguing streptococcal species whose most frequent source of isolation is fermented foods similarly to  Streptococcus thermophilus . However,  S. macedonicus  is closely related to commensal opportunistic pathogens of the  Streptococcus bovis / Streptococcus equinus  complex.    Methodology/Principal Findings  We analyzed the pSMA198 plasmid isolated from the dairy strain  Streptococcus macedonicus  ACA-DC 198 in order to provide novel clues about the main ecological niche of this bacterium. pSMA198 belongs to the narrow host range pCI305/pWV02 family found primarily in lactococci and to the best of our knowledge it is the first such plasmid to be reported in streptococci. Comparative analysis of the pSMA198 sequence revealed a high degree of similarity with plasmids isolated from  Lactococcus lactis  strains deriving from milk or its products. Phylogenetic analysis of the pSMA198 Rep showed that the vast majority of closely related proteins derive from lactococcal dairy isolates. Additionally, cloning of the pSMA198 ori in  L. lactis  revealed a 100% stability of replication over 100 generations. Both pSMA198 and the chromosome of  S. macedonicus  exhibit a high percentage of potential pseudogenes, indicating that they have co-evolved under the same gene decay processes. We identified chromosomal regions in  S. macedonicus  that may have originated from pSMA198, also supporting a long co-existence of the two replicons. pSMA198 was also found in divergent biotypes of  S. macedonicus  and in strains isolated from dispersed geographic locations (e.g. Greece and Switzerland) showing that pSMA198’s acquisition is not a recent event.    Conclusions/Significance  Here we propose that  S. macedonicus  acquired plasmid pSMA198 from  L. lactis  via an ancestral genetic exchange event that took place most probably in milk or dairy products. We provide important evidence that point towards the dairy origin of this species.

Acquisition through Horizontal Gene Transfer of Plasmid pSMA198 by Streptococcus macedonicus ACA-DC 198 Points towards the Dairy Origin of the Species