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Diversity of methicillin-resistant Staphylococcus aureus isolated in a Canadian hospital
O Hammerberg1, H Bialkowska-Hobrzanska, D Gregson
1Department of Microbiology and Infectious Diseases, St. Joseph's Health Centre, London, Ontario, Canada.
Abstract:
Three neonates and three other patients located elsewhere in the hospital became infected with Staphylococcus aureus. Initial automated microdilution susceptibility testing with oxacillin and disk diffusion testing with amoxicillin-clavulanic acid indicated the isolates had borderline oxacillin resistance (MICs 4 micrograms/ml), presumably due to hyperproduction of beta-lactamase. Chromosomal DNA restriction fingerprinting and phage typing revealed the neonatal isolates to be identical; whereas, the other patients were infected with three different strains. Further analysis of the four strains by Southern hybridization with a mecA specific oligoprobe and a quantitative beta-lactamase assay demonstrated that two strains carried the mecA gene (coding for low affinity penicillin-binding protein 2a), and two strains were hyperproducers of beta-lactamase, including one which was mecA gene positive. One strain neither carried the mecA gene nor hyperproduced beta-lactamase. The two mecA gene positive strains displayed oxacillin MICs of 16 micrograms/ml on dilution susceptibility testing in 4% NaCl supplemented Mueller-Hinton agar. Hence, they were considered intrinsically methicillin-resistant Staphylococcus aureus. Both oxacillin and amoxicillin-clavulanic acid MICs were increased on NaCl supplementation. Results of amoxicillin-clavulanic acid disk diffusion susceptibility testing did not correlate with quantitative beta-lactamase production. It is recommended that clinical laboratories do not use amoxicillin-clavulanic acid disk diffusion assays to differentiate suspected borderline resistance due to beta-lactamase hyperproduction from mecA gene expression of PBP-2a since additional mechanisms may account for resistance.
Insights
This study investigated Staphylococcus aureus infections, finding that amoxicillin-clavulanic acid disk diffusion tests may not accurately detect borderline oxacillin resistance. Differentiating beta-lactamase hyperproduction from mecA gene expression requires further methods.
Area of Science:
- Medical Microbiology
- Infectious Diseases
- Antimicrobial Resistance
Background:
- Hospital-acquired Staphylococcus aureus infections can present with complex resistance mechanisms.
- Initial susceptibility testing may not always accurately identify the specific cause of oxacillin resistance.
Purpose of the Study:
- To investigate the mechanisms of borderline oxacillin resistance in Staphylococcus aureus isolates from hospital patients.
- To evaluate the reliability of different susceptibility testing methods in identifying resistance mechanisms.
Main Methods:
- Chromosomal DNA restriction fingerprinting and phage typing were used for isolate characterization.
- Southern hybridization with a mecA specific oligoprobe and quantitative beta-lactamase assays were performed.
- Susceptibility testing with oxacillin and amoxicillin-clavulanic acid was conducted, including NaCl supplementation.
Main Results:
- Two of the four distinct strains carried the mecA gene, conferring methicillin resistance.
- Two strains hyperproduced beta-lactamase, with one also carrying the mecA gene.
- Amoxicillin-clavulanic acid disk diffusion results did not correlate with quantitative beta-lactamase production.
Conclusions:
- Standard amoxicillin-clavulanic acid disk diffusion assays are insufficient for differentiating beta-lactamase hyperproduction from mecA-mediated resistance.
- Clinical laboratories should employ additional methods to accurately determine the mechanisms of oxacillin resistance in Staphylococcus aureus.