Activities of High-Dose Daptomycin, Vancomycin, and Moxifloxacin Alone or in Combination with Clarithromycin or Rifampin in a Novel In Vitro Model of Staphylococcus aureus Biofilm

Biofilm formation is an important virulence factor that allows bacteria to resist host responses and antibacterial agents. The aim of the study was to assess the in vitro activities of several antimicrobials alone or in combination against two Staphylococcus aureus isolates in a novel pharmacokinetic/pharmacodynamic (PK/PD) model of biofilm for 3 days. One methicillin-susceptible S. aureus strain (SH1000) and one methicillin-resistant S. aureus strain (N315) were evaluated in a modified biofilm reactor with polystyrene coupons. Simulated regimens included vancomycin (VAN) plus rifampin (RIF), moxifloxacin (MOX), and high doses (10 mg/kg of body weight/day) of daptomycin (DAP) alone or combined with RIF or clarithromycin (CLA). Against viable planktonic bacteria (PB) and biofilm-embedded bacteria (BB) of SH1000, neither DAP nor MOX alone was bactericidal. In contrast, the combination of DAP or MOX with CLA significantly increased the activity of the two agents against both PB and BB (P < 0.01), and DAP plus CLA reached the limit of detection at 72 h. Against PB of N315, DAP alone briefly achieved bactericidal activity at 24 h, whereas sustained bactericidal activity was observed at 32 h with VAN plus RIF. Overall, only a minimal reduction was observed with both regimens against BB (<2.8 log(10) CFU/ml). Finally, the combination of DAP and RIF was bactericidal against both PB and BB, achieving the limit of detection at 72 h. In conclusion, we developed a novel in vitro PK/PD model to assess the activities of antimicrobials against mature bacterial biofilm. Combinations of DAP or MOX with CLA were the most effective regimens and may represent promising options to treat persistent infections caused by S. aureus biofilms.