Genetic diversity among methicillin-resistant Staphylococcus epidermidis (MRSE).

We selected 106 methicillin-resistant Staphylococcus epidermidis (MRSE) and 22 methicillin-susceptible S. epidermidis (MSSE) hospital isolates--each with a different PFGE pattern--for more detailed documentation of genetic diversity. The 106 MRSE isolates showed extensive variation in the SmaI DNA fragments hybridizing with the DNA probe for mecA, the molecular size of which varied from as low as 20 kb up to over 500 kb. Parallel variation was also observed in the size of DNA fragments hybridizing with the chromosomal genes orfX and gyrA, and this was also observed in MSSE isolates. In contrast, SmaI fragments associated with the housekeeping genes murE and aroE, both located distantly from orfX, showed little size variation. Typing for the mec complex and ccr identified 10 different SCCmec structures and a large number of strains (21 isolates) that were non-typeable. The majority of strains studied (36%) carried a SCCmec type IV-like structure, including strains with non-related PFGE profiles. On the other hand, closely related strains often carried different types of SCCmec. The findings indicate that the acquisition and/or loss of mobile genetic elements, including various structural types of SCCmec, may occur frequently in the vicinity of the orfX gene on the S. epidermidis chromosome.