Oxacillin-susceptible methicillin-resistant Staphylococcus aureus septic bursitis identified following clinical...

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.

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.

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).

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 using an antibiotic of a different class, trimethoprim-sulfamethoxazole, for which the isolate tested sensitive.