Oxacillin-susceptible methicillin-resistant Staphylococcus aureus septic bursitis identified following clinical failure with β-lactam therapy: a case report

Background: Oxacillin-susceptible methicillin-resistant Staphylococcus aureus (OS-MRSA) is a phenotype of S. aureus that presents as methicillin susceptible on traditional screening tests but may have a resistant phenotype, leading to clinical failure. Methods: An isolate of S. aureus screened negative for methicillin resistance in a case of septic bursitis in a 75-year-old female. After failure to improve on cefazolin, a second aspirate was performed and sent for testing. The second sample strain retrieved had an additional PBP2a lateral flow performed, demonstrating the presence of mecA products. Results: Both S. aureus isolates proved to be identical on pulsed-field gel electrophoresis and had faint positive PBP2a bands on lateral flow, despite screening negative for methicillin resistance on oxacillin and cefoxitin testing. A point mutation was found in the mecA start codon, which could explain the OS-MRSA phenotype. Neither oxacillin nor cefoxitin screening tests were positive following a cefoxitin induction assay that was used locally to promote a methicillin-resistant phenotype in small-colony variant S. aureus (SCV). Conclusions: OS-MRSA is a rare but documented phenotype of S. aureus that has the potential to escape typical screening. Simple mutations may lead to reversal under selection pressure. Failure to respond to standard-of-care therapy for methicillin-sensitive S. aureus should prompt additional testing. Such methods could include a PBP2a immunoassay with high sensitivity. The patient recovered by utilizing an antibiotic of a different class, trimethoprim-sulfamethoxazole, for which the isolate tested sensitive.