Heterogeneous oxacillin-resistant phenotypes and production of PBP2A by oxacillin-susceptible/mecA-positive MRSA strains from Africa

Objectives: Recent surveillance of MRSA colonizing patients and healthcare workers in two African countries (Angola and São Tomé and Príncipe) reported the frequent recovery of oxacillin-susceptible MRSA (OS-MRSA): Staphylococcus aureus strains that gave positive results with the mecA DNA probe, but had low oxacillin MIC values characteristic of susceptible S. aureus. This apparent dissociation of the drug-resistant phenotype from mecA-the primary genetic determinant of resistance-prompted us to perform a more detailed analysis on nine of the African OS-MRSA strains. Methods: Oxacillin MIC values were determined by Etest and population analysis profiles with and without induction of the stringent stress response by mupirocin. Biochemical profiling using SDS-PAGE followed by western blotting was used for the detection of PBP2A protein produced. Results: Cultures of the African MRSA strains (ST88-IVa and ST8-V) showed heterogeneous oxacillin resistance in which the majority of cells exhibited low oxacillin MICs (≤0.75 mg/L), but highly resistant subpopulations were also present with oxacillin MIC values up to several hundred mg/L and with frequencies of 10^-4 to 10^-6. The same strains after induction of the stringent stress response by mupirocin 'converted' the heterogeneous phenotypes into a more homogeneous and higher level resistance. After induction by oxacillin and mupirocin, each of the nine African OS-MRSA strains produced PBP2A-the protein product of mecA. Conclusions: The resistant phenotype of OS-MRSA resembles the phenotypes of historically early MRSA clones. The nature of genetic determinants responsible for the heterogeneous phenotypes of OS-MRSA remains to be determined.