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

Abstract

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.

FIG. 1.

Activity of DAP at 10 mg/kg/day q24h and 400 mg MOX daily, alone or in combination with CLA at 250 mg q12h against SH1000. (A) Planktonic bacteria; (B) biofilm-embedded bacteria. White circles, DAP; black inversed triangles, MOX; white triangles, DAP plus CLA; black squares, MOX plus CLA; black circles, GC, growth control (organism growth with no drug added).

FIG. 2.

SEM images of coupon surfaces to assess the presence and structure of the matrix of a SH1000 biofilm. Images were collected at 2,000× magnification. (A) Before any drug exposure; (B) after 72 h of daptomycin exposure; (C) after 72 h of daptomycin plus clarithromycin exposure; (D) after 72 h of moxifloxacin plus clarithromycin exposure.

FIG. 3.

Activity of DAP at 10 mg/kg/day q24h, alone or in combination with RIF, and of VAN in combination with RIF against N315. (A) Planktonic bacteria; (B) biofilm-embedded bacteria. White circles, DAP; black inversed triangles, VAN plus RIF; white triangles, DAP plus RIF; black circles, GC, growth control (organism growth with no drug added).