Activities of fosfomycin, tigecycline, colistin, and gentamicin against extended-spectrum-β-lactamase-producing Escherichia coli in a foreign-body infection model

Abstract

Limited antimicrobial agents are available for the treatment of implant-associated infections caused by fluoroquinolone-resistant Gram-negative bacilli. We compared the activities of fosfomycin, tigecycline, colistin, and gentamicin (alone and in combination) against a CTX-M15-producing strain of Escherichia coli (Bj HDE-1) in vitro and in a foreign-body infection model. The MIC and the minimal bactericidal concentration in logarithmic phase (MBC(log)) and stationary phase (MBC(stat)) were 0.12, 0.12, and 8 μg/ml for fosfomycin, 0.25, 32, and 32 μg/ml for tigecycline, 0.25, 0.5, and 2 μg/ml for colistin, and 2, 8, and 16 μg/ml for gentamicin, respectively. In time-kill studies, colistin showed concentration-dependent activity, but regrowth occurred after 24 h. Fosfomycin demonstrated rapid bactericidal activity at the MIC, and no regrowth occurred. Synergistic activity between fosfomycin and colistin in vitro was observed, with no detectable bacterial counts after 6 h. In animal studies, fosfomycin reduced planktonic counts by 4 log(10) CFU/ml, whereas in combination with colistin, tigecycline, or gentamicin, it reduced counts by >6 log(10) CFU/ml. Fosfomycin was the only single agent which was able to eradicate E. coli biofilms (cure rate, 17% of implanted, infected cages). In combination, colistin plus tigecycline (50%) and fosfomycin plus gentamicin (42%) cured significantly more infected cages than colistin plus gentamicin (33%) or fosfomycin plus tigecycline (25%) (P < 0.05). The combination of fosfomycin plus colistin showed the highest cure rate (67%), which was significantly better than that of fosfomycin alone (P < 0.05). In conclusion, the combination of fosfomycin plus colistin is a promising treatment option for implant-associated infections caused by fluoroquinolone-resistant Gram-negative bacilli.

Fig 1

Time-kill curves with 0.5, 1, and 4× the MIC of tigecycline (TIG), colistin (COL), fosfomycin (FOS), and gentamicin (GEN) against E. coli in log growth phase (inoculum, 10⁶ CFU/ml). Values are means ± SD. The experiments were performed in triplicate. GC, growth control.

Fig 2

Time-kill curves with 0.5× the MIC of tigecycline (TIG), colistin (COL), fosfomycin (FOS), and gentamicin (GEN) in combinations against E. coli in log growth phase (inoculum, 10⁶ CFU/ml). Values are means ± SD. The experiments were performed in triplicate. GC, growth control.

Fig 3

Activities against planktonic bacteria in cage fluid aspirated during treatment (i.e., day 5; open bars) and 5 days after end of treatment (i.e., day 10; closed bars). In each group, fluid from 12 cages (from 3 animals) was investigated. The y axis shows log₁₀ CFU/ml in aspirated cage fluid, expressed as means ± standard deviations (SD).

Fig 4

Rate of cure of cage-associated infection. The values are numbers of cage cultures without growth of E. coli divided by the total number of cages in the treatment group (n = 12). Significant differences compared to untreated controls are indicated with asterisks (*, P < 0.05; **, P < 0.01).