In vivo deletion of the methicillin resistance mec region from the chromosome of Staphylococcus aureus strains

Two sets of Staphylococcus aureus isolates recovered from two patients exhi bited similar susceptibility profiles except for oxacillin susceptibility ( MSSA) or resistance (MRSA). SmaI macrorestriction and inter-IS256 PCR analy sis showed patterns closely related to the Belgian epidemic MRSA clone 1 in each pair of MSSA/MRSA strains. Loss of one large SmaI DNA fragment and co ncurrent gain of a smaller fragment in the MSSA isolates was observed. The mecA sequence present in the MRSA was absent in the MSSA variant. Therefore , in vivo deletion of the mec region may occur in some lineages of S. aureu s more frequently than previously thought.