title: Diversity of methicillin-resistant Staphylococcus aureus isolated in a Canadian hospital.
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.
authors.affiliation: Department of Microbiology and Infectious Diseases, St. Joseph's Health Centre, London, Ontario, Canada.
authors.forename: O
authors.initials: O
authors.lastname: Hammerberg
authors.forename: H
authors.initials: H
authors.lastname: Bialkowska-Hobrzanska
authors.forename: D
authors.initials: D
authors.lastname: Gregson
authors.forename: K
authors.initials: K
authors.lastname: McGhie
authors.forename: R
authors.initials: R
authors.lastname: Behme
mesh_terms: D000328:Adult; D000658:Amoxicillin / Q000494:pharmacology; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D015373:Bacterial Typing Techniques; D001483:Base Sequence; D002170:Canada; D019818:Clavulanic Acid; D002969:Clavulanic Acids / Q000494:pharmacology; D003428:Cross Infection / Q000382:microbiology; D004269:DNA, Bacterial / Q000032:analysis*; D006801:Humans; D007231:Infant, Newborn; D016106:Methicillin Resistance / Q000235:genetics; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D010068:Oxacillin / Q000494:pharmacology; D013203:Staphylococcal Infections / Q000188:drug therapy / Q000382:microbiology; D013211:Staphylococcus aureus / Q000187:drug effects / Q000201:enzymology / Q000235:genetics; D001618:beta-Lactamases / Q000096:biosynthesis
chemical_list: D000900:Anti-Bacterial Agents; D002969:Clavulanic Acids; D004269:DNA, Bacterial; D019818:Clavulanic Acid; D000658:Amoxicillin; D001618:beta-Lactamases; D010068:Oxacillin