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 invitro 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 24h and viable counting of crushed bone blocks was used to determine bacterial survival. Challenge of 72h biofilms with gentamicin and daptomycin for 24h demonstrated that only concentrations of 10 times the clinical peak serum target levels (100mgl−1 gentamicin and 1000mg l−1 daptomycin) resulted in significant reductions in cell viability compared to controls. Once daily dosing over 7days resulted in ≥3 log reductions in cell numbers by 48h. Thereafter no significant reduction was achieved, although emergence of resistance was suppressed. Determination of antibiotic concentration in bone blocks over 7days indicated that neither agent was able to consistently reach levels in bone of >10% of the original dose. The model was, therefore, able to demonstrate the challenges posed by biofilm growth on and within bone. Significance and Impact of the Study: The majority of studies of antibiotic efficacy in the treatment of chronic osteomyelitis are carried out in animals. We developed an invitro model of Staphylococcus aureus infection of bone to evaluate the ability of antibiotics to eradicate mature biofilms on surfaces analogous to necrotic bone. The results demonstrated the difficulties which occur in osteomyelitis treatment, with only very high concentrations of antibiotic able to penetrate the bone sufficiently to reduce bacterial survival whilst still failing to eradicate biofilms. This model could be of use in initial screening of novel compounds intended for use in the treatment of osteomyelitis