Background: Our research group recently showed that SCCmec type III is by far the most common type amongst methicillin-resistant coagulase-negative staphylococci (CoNS) of animal origin, in particular isolates within the Staphylococcus sciuri-group (S. sciuri, S. vitulinus, S. lentus and S. fleurettii). Type III cassettes are the largest of all known SCCmec elements and could to be the progenitor of other SCCmec elements since they harbour mec complex type A with intact regulatory genes. The objective of this study was to investigate trends in the evolution of SCCmec III elements. Variations in mecA and ccrB were used as markers to study evolution and possible transfer of SCCmec III across distinct staphylococcal species.

Materials and Methods: A collection of genetically and epidemiologically diverse animal and human S. aureus (n=5), S. fleurettii (n=5), S. capitis (n=4), S. pseudintermedius (n=2), S. vitulinus (n=2), S. sciuri (n=2), S. haemolyticus (n=1) and S. equorum (n=1) harbouring SCCmec III elements was subjected to partial sequence analysis of mecA and ccrB according to previously published protocols (Oliveira et al., JAC, 2006; Stephens et al., AAC, 2007). The sequences were analyzed by neighbour-joining analysis to assess phylogenetic relationships between and within species.

Results: Most (78%) ccrB sequences were identical and corresponded to the prototype allele 300 found in all S. aureus SCCmec III described in the current database (www.ccrbtyping.net). Four CoNS isolates (S. sciuri, S. equorum, S. haemolyticus and S. vitulinus) had ccrB sequences displaying only 75-80% similarity to allele 300, and three S. fleurettii isolates yielded no ccrB bands. mecA was highly conserved overall (>99% similarity), yet 10 alleles were defined based on variation in 11 nucleotide positions. As for ccrB, species-specific clustering of mecA sequences was not observed as exemplified by mecA allele 5 being present in four species (S. pseudintermedius, S. sciuri, S. haemolyticus and S. vitulinus). Similarly, mecA allele 1 was found in three species (S. aureus, S. capitis and S. pseudintermedius). Although various combinations of mecA and ccrB alleles were detected, seven isolates representing S. aureus (n=2), S. pseudintermedius (n=2) and S. capitis (n=3) had the same combination of ccrB and mecA alleles, suggesting interspecies horizontal transfer of SCCmec III.

Conclusion: The occurrence in distinct staphylococcal species of SCCmec type III that are undistinguishable on the basis of ccrB and mecA sequencing suggests that this large genetic element has been transferred horizontally between distantly related species within the genus. Ongoing studies on sequence diversity of the SCCmec junkyard regions in these isolates will be used to further test this hypothesis.