Comparative analysis of pSMA198 found in Streptococcus macedonicus ACA-DC 198, the first streptococcal plasmid of the pCI305/pWV02 family of theta-replicating replicons

Here we analyze pSMA198, the first plasmid isolated from Streptococcus macedonicus ACA-DC 198, and we attempt to clarify the route of its original acquisition. Based on the similarity profiles of the plasmid’s replication initiation protein (Rep) and its origin of replication (ori), pSMA198 was found to be a novel member of the pCI305/pWV02 family of theta-replicating plasmids. The pCI305/pWV02 family consists of plasmids of narrow host range that are mainly found in lactococcal species. Comparative analysis of the pSMA198 revealed a high degree of similarity with plasmids pSK11b, pVF22 and pIL5 over its replication backbone, its mobilization backbone and most of its length, respectively. All these three plasmids have been isolated from Lactococcus lactis strains deriving from milk or its products supporting that S. macedonicus acquired pSMA198 from the latter species and that this acquisition took place in the dairy environment. Both pSMA198 and the chromosome of S. macedonicus exhibit a high degree of pseudogenes, indicating that they must have evolved under the same gene decay processes. Furthermore, we were able to determine chromosomal regions that may have originated from pSMA198, also supporting a long co-existence of the two replicons. In addition, pSMA198 is carried by S. macedonicus strains segregated in five different genotypes by pulsed-field gel electrophoresis (PFGE), showing that pSMA198’s acquisition is not a recent event. We propose that our overall analysis of pSMA198 points towards the habituation of S. macedonicus ACA-DC 198 to the dairy environment