Optimal Local Dosage of Housekeeping Antibiotics to Inhibit S. Aureus without Inducing Osteoblast Necrosis: In Vitro Study

Staphylococcus aureus is commonly seen in orthopaedic implant related infections. A high local concentration of antibiotics is toxic to osteoblasts, thereby inhibiting bone formation. Account has to be taken while designing antibiotic coated implants and delivering antibiotics locally. Cefazolin, vancomycin and gentamicin are usually used. However, the correlations between such antibiotics and osteoblast activity have not been well studied in the optimal dosage to inhibit that bacterium without inducing cytotoxic effect. This study aims to investigate the tolerance of osteoblasts against such antibiotics in-vitro. Osteoblasts (SaOs2) were cultured in antibiotic free medium and then exposed to concentrations of each antibiotics between 10,000ug/ml and 0.001ug/ml. Viable cells at 24 hours were counted using Thiazolyl Blue Tetrazolium Bromide (MTT) assay. By using the same concentration gradient, minimal inhibition concentration of such antibiotics to S. aureus was studied. In cytotoxicity test, osteoblasts die as the concentration of antibiotics increases to 10,000ug/ml, whereas all the cells can survive as the concentration reduces to 1000ug/ml or below. In minimal inhibition concentration testing, it suggests 0.625ug/ml of gentanmicin and/or vancomycin is able to inhibit S. aureus activity. For cefazolin the concentration even reduces to 0.15625ug/ml. According to guideline, the recommended daily dosages of cefazolin, gentanmicin and vancomycin are 35.7ug/ml, 3.75ug/ml and 17.86ug/ml, respectively. By using the in-vitro results, these clinical dosages do not induce any toxic effect to osteoblasts and are also capable to inhibit S. aureus activity.