Investigation of the efficacy of high-dose flucloxacillin therapy for borderline oxacillin-resistant (BORSA) Staphylococcus aureus infections of Galleria mellonella larvae

Introduction. Infections caused by borderline oxacillin-resistant Staphylococcus aureus (BORSA) isolates can be misdiagnosed as methicillin-resistant Staphylococcus aureus (MRSA) or methicillin-sensitive Staphylococcus aureus (MSSA), resulting in problems with antibiotic therapy.

Gap statement. There is a paucity of data available describing the effectiveness of beta-lactam therapy on BORSA infections.

Aim. Compare the efficacy of oxacillin, flucloxacillin and high-dose flucloxacillin therapy on MSSA, MRSA and BORSA infections in vivo using larvae of the greater wax moth, Galleria mellonella.

Methodology. Clinical isolates of MSSA, MRSA and BORSA were used to infect larvae, and the effect of administration of oxacillin and flucloxacillin, including high-dose flucloxacillin, on survival was compared.

Results. A typical dose of oxacillin or flucloxacillin administered to G. mellonella larvae infected with an MSSA strain resulted in effective therapy. The same doses given to larvae infected with MRSA or BORSA isolates conferred no therapeutic benefit. Similarly, treatment with high-dose flucloxacillin of larvae infected with the same isolates also offered no therapeutic benefit after 4 days.

Conclusion. Treatment of BORSA infections of G. mellonella with high-dose flucloxacillin was not successful, implying that BORSA infections should be treated with non-penicillin antibiotics such as vancomycin. The study also confirmed the utility of using G. mellonella larvae for studying the efficacy of different antibiotic therapies against S. aureus infections.