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Published on: September 5, 2013
The NaHCO3-Responsive Phenotype in Methicillin-Resistant Staphylococcus aureus (MRSA) Is Influenced by mecA Genotype
Selvi C Ersoy1, Adhar C Manna2, Richard A Proctor3
1The Lundquist Institute, Torrance, California, USA.
Abstract:
Methicillin-resistant Staphylococcus aureus (MRSA) strains are a leading cause of many invasive clinical syndromes, and pose treatment difficulties due to their in vitro resistance to most β-lactams on standard laboratory testing. A novel phenotype frequently identified in MRSA strains, termed 'NaHCO3-responsiveness', is a property whereby strains are susceptible in vitro to many β-lactams in the presence of NaHCO3. Specific mecA genotypes, repression of mecA/PBP2a expression and perturbed maturation of PBP2a by NaHCO3 have all been associated with this phenotype. The aim of this study was to define the relationship between specific mecA genotypes and PBP2a substitutions, on the one hand, with NaHCO3-responsiveness in vitro. Mutations were made in the mecA ribosomal binding site (RBS -7) and at amino acid position 246 of its coding region in parental strains MW2 (NaHCO3-responsive) and C36 (NaHCO3- nonresponsive) to generate 'swap' variants, each harboring the other's mecA-RBS/coding region genotypes. Successful swaps were confirmed by both sequencing, as well as predicted swap of in vitro penicillin-clavulanate susceptibility phenotypes. MW2 swap variants harboring the nonresponsive mecA genotypes became NaHCO3-nonresponsive (resistant to the β-lactam, oxacillin [OXA]), in the presence of NaHCO3. Moreover, these swap variants had lost NaHCO3-mediated repression of mecA/PBP2a expression. In contrast, C36 swap variants harboring the NaHCO3-responsive mecA genotypes remained NaHCO3-nonresponsive phenotypically, and still exhibited nonrepressible mecA/PBP2a expression. These data demonstrate that in addition to the mecA genotype, NaHCO3-responsiveness may also depend on strain-specific genetic backgrounds.
Insights
NaHCO3-responsiveness in MRSA is linked to specific mecA genotypes and PBP2a alterations. Strain-specific genetic backgrounds also influence this phenotype, impacting beta-lactam susceptibility in vitro.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Methicillin-resistant Staphylococcus aureus (MRSA) presents significant treatment challenges due to resistance to beta-lactam antibiotics.
- A subset of MRSA strains exhibit 'NaHCO3-responsiveness,' a phenotype conferring in vitro susceptibility to beta-lactams in the presence of sodium bicarbonate.
- This responsiveness is associated with specific mecA genotypes, altered mecA/PBP2a expression, and perturbed PBP2a maturation.
Purpose of the Study:
- To elucidate the relationship between specific mecA genotypes, PBP2a substitutions, and NaHCO3-responsiveness in MRSA.
- To investigate the role of mecA ribosomal binding site (RBS) and coding region mutations in conferring NaHCO3-responsiveness.
Main Methods:
- Generated 'swap' variants by exchanging mecA-RBS/coding region genotypes between NaHCO3-responsive (MW2) and nonresponsive (C36) MRSA strains.
- Confirmed successful genetic swaps using DNA sequencing.
- Assessed in vitro susceptibility to beta-lactams (oxacillin) in the presence of NaHCO3 and evaluated mecA/PBP2a expression levels.
Main Results:
- MW2 swap variants with nonresponsive mecA genotypes lost NaHCO3-responsiveness and showed resistance to oxacillin, along with loss of NaHCO3-mediated repression of mecA/PBP2a expression.
- C36 swap variants with responsive mecA genotypes remained nonresponsive phenotypically and exhibited nonrepressible mecA/PBP2a expression.
- These findings indicate that while mecA genotype is crucial, strain-specific genetic factors also contribute to NaHCO3-responsiveness.
Conclusions:
- The mecA genotype, specifically mutations in the RBS and coding region, plays a significant role in NaHCO3-responsiveness in MRSA.
- Strain-specific genetic backgrounds are critical determinants of NaHCO3-responsiveness and the regulation of mecA/PBP2a expression.
- Understanding these genetic interactions is vital for developing novel therapeutic strategies against MRSA infections.
