High activity of Fosfomycin and Rifampin against methicillin-resistant staphylococcus aureus biofilm in vitro and in an experimental foreign-body infection model

Abstract

Increasing antimicrobial resistance reduces treatment options for implant-associated infections caused by methicillin-resistant Staphylococcus aureus (MRSA). We evaluated the activity of fosfomycin alone and in combination with vancomycin, daptomycin, rifampin, and tigecycline against MRSA (ATCC 43300) in a foreign-body (implantable cage) infection model. The MICs of the individual agents were as follows: fosfomycin, 1 μg/ml; daptomycin, 0.125 μg/ml; vancomycin, 1 μg/ml; rifampin, 0.04 μg/ml; and tigecycline, 0.125 μg/ml. Microcalorimetry showed synergistic activity of fosfomycin and rifampin at subinhibitory concentrations against planktonic and biofilm MRSA. In time-kill curves, fosfomycin exhibited time-dependent activity against MRSA with a reduction of 2.5 log10 CFU/ml at 128 × the MIC. In the animal model, planktonic bacteria in cage fluid were reduced by <1 log10 CFU/ml with fosfomycin and tigecycline, 1.7 log10 with daptomycin, 2.2 log10 with fosfomycin-tigecycline and fosfomycin-vancomycin, 3.8 log10 with fosfomycin-daptomycin, and >6.0 log10 with daptomycin-rifampin and fosfomycin-rifampin. Daptomycin-rifampin cured 67% of cage-associated infections and fosfomycin-rifampin cured 83%, whereas all single drugs (fosfomycin, daptomycin, and tigecycline) and rifampin-free fosfomycin combinations showed no cure of MRSA cage-associated infections. No emergence of fosfomycin resistance was observed in animals; however, a 4-fold increase in fosfomycin MIC (from 2 to 16 μg/ml) occurred in the fosfomycin-vancomycin group. In summary, the highest eradication of MRSA cage-associated infections was achieved with fosfomycin in combination with rifampin (83%). Fosfomycin may be used in combination with rifampin against MRSA implant-associated infections, but it cannot replace rifampin as an antibiofilm agent.

FIG 1

Evaluation of synergistic activity of fosfomycin (FOF) and rifampin (RIF) by microcalorimetry on planktonic (A) and biofilm (B) MRSA. Numbers above curves represent drug concentrations (μg/ml). Bacterial growth-related heat was suppressed by subinhibitory concentrations of fosfomycin (2 μg/ml for planktonic MRSA and 1,024 μg/ml for biofilm MRSA), combined with a subinhibitory concentration of rifampin (0.25× the MHIC).

FIG 2

Time-kill studies for fosfomycin against MRSA at 1×, 8×, 32×, 64×, and 128× the MIC. The fosfomycin MIC was 1 μg/ml. The dotted line indicates a 3-log-CFU reduction. GC, growth control.

FIG 3

Antimicrobial activity against planktonic MRSA. (A) Bacterial counts during therapy (closed bars) and 5 days after the end of therapy (open bars), compared to the initial bacterial counts on day 3 (before treatment). Positive values denote net bacterial growth, and negative values denote net bacterial killing. Values represent medians and interquartile ranges (IQR). Asterisks indicate therapy regimens exhibiting significant differences in bacterial load during therapy versus 5 days after the end of therapy (P < 0.05). (B) Clearance rate of planktonic MRSA in aspirated cage fluids, during therapy (closed bars), and 5 days after the end of therapy (open bars). For each treatment group, 12 cages were used (i.e., 3 animals each implanted with 4 cages). The numbers of MRSA-free cage fluid samples divided by the total number of cages in each therapy group are indicated on the bars. Control, untreated animals; DAP, daptomycin; FOF, fosfomycin; RIF, rifampin; TGC, tigecycline; VAN, vancomycin.

FIG 4

Antimicrobial activity against biofilm MRSA. For each treatment group, 3 animals each implanted with 4 cages were used. Numbers denote cure rates of cage-associated MRSA infections. Numbers of sterile explanted cages divided by the total number of cages in each therapy group are indicated on the bars. Control, untreated animals; DAP, daptomycin; FOF, fosfomycin; RIF, rifampin; TGC, tigecycline; VAN, vancomycin.