Identification of a Novel Transposon (Tn 6072 ) and a Truncated Staphylococcal Cassette Chromosome mec Element in Methicillin-Resistant Staphylococcus aureus ST239

A novel composite transposon (Tn 6072 ) resembling staphylococcal cassette chromosome mercury (SCC Hg ) was identified in a collection of sequence type (ST) 239 methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) isolates from Romanian hospitals. Tn 6072 is homologous to the 5′ region of SCC Hg found in staphylococcal cassette chromosome mec (SCC mec ) type III prototype strain 85/2082 but lacks the characteristic mer operon. SCC Hg has previously been reported to integrate downstream of orfX , at the same chromosomal location as SCC mec . Tn 6072 , by contrast, is demarcated by two IS 431 elements, flanked by 8-bp direct repeats, and inserted upstream of the origin of replication, within an open reading frame homologous to SAR2700 of S. aureus strain MRSA252. Analysis of a geographically and temporally diverse collection of 111 strains from the ST239 clonal group uncovered 11 additional strains harboring Tn 6072 , demonstrating a lineage-specific insertion pattern. Complete sequence analysis of the SCC mec regions of two representative Romanian strains (BK16704, BK16691) revealed two additional novel structures derived from a type III SCC mec background. BK16704 possesses an SCC mec 3A.1.4 structure, with an IS 256 insertion downstream of the right chromosomal junction. In contrast, the SCC mec element of BK16691 is truncated downstream of the mec gene complex, with a 24-kb deletion encompassing the right chromosomal junction and an inverted downstream IS 256 element. This structure, tentatively named “ψSCC mec 16691 ,” confers methicillin resistance but lacks most of the J1/J2 region, including the ccr gene complex. Taken together, these findings provide evidence for the continuing evolution of SCC elements, as well as the ST239 clonal group.