Determination of the chromosomal relationship between mecA and gyrA in methicillin-resistant coagulase-negative staphylococci.

mecA, the gene that mediates methicillin resistance, and its accompanying mec locus DNA, insert near the gyrA gene in Staphylococcus aureus. To investigate whether there is a similar relationship between mecA and gyrA in coagulase-negative staphylococci (CNS), mecA- and gyrA-specific DNA fragments were used to probe methicillin-resistant isolates of Staphylococcus epidermidis (MRSE) (n = 11) and Staphylococcus haemolyticus (MRSH) (n = 11). The gyrA probe hybridized to the same SmaI DNA fragment as the mecA probe in all strains tested. However, since the size of the SmaI fragments containing mecA and gyrA varied from 73 to 600 kb, the distance between the two genes was determined more precisely. Cloned mecA or gyrA fragments plus vector sequences each containing a SmaI site were introduced into the chromosome of three isolates each of MRSE and methicillin-resistant S. aureus (MRSA), and the sizes of the generated SmaI fragments were determined by pulsed-field gel electrophoresis. The distance between gyrA and mecA was found to be between 38 and 42 kb in both MRSE and MRSA, and the two genes were in the same relative orientation in all strains. Restriction fragment length polymorphism (RFLP) patterns around the gyrA gene in CNS were identical, but species specific, for all 10 MRSE and 10 MRSH isolates examined. In contrast, 8 of 11 methicillin-susceptible S. epidermidis isolates and 7 of 7 methicillin-susceptible S. haemolyticus isolates had different gyrA RFLP patterns. These data show that mecA is site and orientation specific, relative to gyrA, in both MRSE and MRSA. In addition, the local environment around gyrA in methicillin-resistant CNS, in contrast to methicillin-susceptible isolates, is similar, suggesting clonality or the requirement for specific DNA sequences with which the mec complex must interact for chromosomal integration to occur.