An in vitro biofilm model of Staphylococcus aureus infection of bone

Chronic osteomyelitis is difficult to treat, with biofilm growth and the diffusion barrier to antibiotics presented by bone contributory factors. The aim of this study was to develop and evaluate an in vitro model of osteomyelitis. A bioluminescent strain of Staphylococcus aureus was grown in bone blocks made from bovine femur. Light output was insufficient for detection of bacterial cells within bone by 24 h and viable counting of crushed bone blocks was used to determine bacterial survival. Challenge of 72 h biofilms with gentamicin and daptomycin for 24 h demonstrated that only concentrations of ten times the clinical peak serum target levels (100 mg l-1 gentamicin and 1000 mg l-1 daptomycin) resulted in significant reductions in cell viability compared to controls. Once daily dosing over seven days resulted in ≥3 log reductions in cell numbers by 48 h. Thereafter no significant reduction was achieved, although emergence of resistance was suppressed. Determination of antibiotic concentration in bone blocks over seven days indicated that neither agent was able to consistently reach levels in bone of greater than 10% of the original dose. The model was, therefore, able to demonstrate the challenges posed by biofilm growth on and within bone.