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Conversion of oxacillin-resistant staphylococci from heterotypic to homotypic resistance expression
J E Finan1, A E Rosato, T M Dickinson
1Department of Medicine, Medical College of Virginia, Virginia Commonwealth University, Richmond, Virginia 23298-0049, USA.
Abstract:
Staphylococci that acquire the mecA gene are usually resistant to beta-lactam antibiotics (methicillin or oxacillin resistance). mecA encodes a penicillin-binding protein (PBP 2a) that has a reduced affinity for beta-lactams. In some isolates with methicillin or oxacillin resistance, only a small proportion (< or =0.1%) of the population expresses resistance to > or =10 microg of oxacillin per ml (heterotypic resistance [HeR]), while in other isolates most of the population expresses resistance (homotypic resistance [HoR]). In the present study, growth of Staphylococcus aureus or Staphylococcus epidermidis strains with HeR in concentrations of oxacillin (0.3 to 0.7 microg/ml) that produced a fall or a lag in optical density converted the strains from the HeR to the HoR phenotype. The conversion from the HeR to the HoR phenotype appeared to be due to the selection of a highly resistant mutant population, as determined by fluctuation analysis and the failure of populations with HoR to revert to HeR after 60 generations of growth in antibiotic-free media. The frequencies of conversion were as high as 10(-3) to 10(-2). Conversion to HoR required an intact mecA gene and an increase in the level of mecA transcription since no highly resistant subpopulation could be selected after growth in oxacillin when mecA transcription was constitutively repressed or when mecA had been inactivated. In addition, in both S. epidermidis and S. aureus the level of resistance to vancomycin, which also acts directly on the staphylococcal cell wall, was greater among convertants with HoR than their isogenic parents. The conversion of a population from HeR to HoR involves the selection of a mutation(s) that occurs at a high frequency and most likely requires abundant PBP 2a.
Insights
Methicillin-resistant Staphylococcus strains with heterotypic resistance (HeR) can rapidly convert to homotypic resistance (HoR) under oxacillin pressure. This conversion, driven by high-frequency mutations and requiring the mecA gene, increases resistance to other antibiotics like vancomycin.
Area of Science:
- Microbiology
- Molecular Biology
- Antibiotic Resistance
Background:
- Staphylococci acquiring the mecA gene exhibit resistance to beta-lactam antibiotics.
- Methicillin or oxacillin resistance is mediated by the mecA gene encoding penicillin-binding protein 2a (PBP 2a).
- Strains display heterotypic resistance (HeR) with low-level resistance expression or homotypic resistance (HoR) with high-level expression.
Purpose of the Study:
- To investigate the conversion of Staphylococcal strains from heterotypic resistance (HeR) to homotypic resistance (HoR).
- To elucidate the mechanisms and genetic requirements underlying this phenotypic switch.
- To assess the impact of this conversion on resistance to other antibiotics, such as vancomycin.
Main Methods:
- Culturing Staphylococcus aureus and Staphylococcus epidermidis strains with HeR under specific oxacillin concentrations.
- Employing fluctuation analysis to determine mutation frequencies and assess the stability of the HoR phenotype.
- Analyzing the role of the mecA gene and its transcription levels in the conversion process.
Main Results:
- Growth in sub-inhibitory oxacillin concentrations converted HeR strains to the HoR phenotype.
- Conversion was attributed to the selection of highly resistant mutant populations at frequencies up to 10(-2).
- The conversion required an intact mecA gene and increased mecA transcription.
- HoR convertants exhibited increased vancomycin resistance compared to their HeR parent strains.
Conclusions:
- Staphylococcal populations can readily transition from heterotypic to homotypic resistance under antibiotic selection.
- This conversion is a high-frequency mutational event dependent on mecA gene function and expression.
- The switch to HoR can lead to collateral increases in resistance to other critical antibiotics, impacting treatment strategies.