Intraosteoblastic activity of levofloxacin and rifampin alone and in combination against clinical isolates of meticillin-susceptible Staphylococcus aureus causing prosthetic joint infection

Background: Staphylococcus aureus may invade and persist intracellularly in prosthetic joint infections (PJIs). Despite optimized treatments with levofloxacin plus rifampin, the intracellular reservoir may lead to infection relapse. This study assessed the intracellular activity of levofloxacin and rifampin in an in-vitro model of human osteoblastic infection.

Methods: Ten meticillin-susceptible S. aureus strains were used to infect osteoblastic MG63 cells. Osteoblasts were challenged with rifampin and levofloxacin at cortical and cancellous bone concentrations. Efficacy was measured as the intracellular counts of colony-forming units (log₁₀CFU) compared with untreated controls. The emergence of small colony variants (SCVs) was determined, and the results were stratified according to the patient's prognosis (six cured and four with persistence/relapse).

Results: All regimes led to a significant decrease in CFU count compared with controls (1-2 log₁₀CFU). Levofloxacin was the most effective treatment at both cortical and cancellous bone concentrations (-2.4 to -1.9 log₁₀CFU, respectively). The addition of rifampin to levofloxacin did not improve performance (-1.9 log₁₀CFU for cortical concentration and -1.8 log₁₀ CFU for cancellous concentration). An increase in SCVs was observed in the presence of rifampin. The efficacy of antimicrobials was higher and the formation of SCVs was lower against strains belonging to PJIs with a favourable outcome.

Conclusions: Levofloxacin plus rifampin had good intracellular activity against S. aureus. However, from the intracellular perspective, the addition of rifampin to levofloxacin showed no benefit but could account for an increased number of SCVs.