Fidaxomicin inhibits spore production in Clostridium difficile

Abstract

Fidaxomicin (FDX) is a novel antimicrobial agent with narrow-spectrum and potent bactericidal activity against Clostridium difficile. In recent clinical trials, FDX was superior to vancomycin in preventing recurrences of C. difficile infection. A possible mechanism of reducing recurrence may be through an inhibitory effect on sporulation. The effect of FDX and its major metabolite, OP-1118, on C. difficile growth and sporulation kinetics was compared with that of vancomycin, metronidazole, and rifaximin. Drugs at subminimum inhibitory concentrations (sub-MICs) were added to cells at an early stationary phase of growth; this was followed by collection of cells at various intervals for quantitation of total viable cell and heat-resistant spore counts on taurocholate-containing media. The effect of the drugs at 2-2.5× MIC on the expression of sporulation genes in C. difficile was also compared using quantitative reverse-transcriptase polymerase chain reaction. Both FDX and OP-1118 (1/4× MIC) inhibited sporulation when added to early-stationary-phase cells in C. difficile strains, including the epidemic NAP1/BI/027 strain. In contrast, vancomycin, metronidazole, and rifaximin (at similar sub-MICs) did not inhibit sporulation. The number of spores following treatment with comparator drugs increased to the same level as the no-drug control treatment. Expression of mother cell-specific (spoIIID) and forespore-specific (spoIIR) sporulation genes also was inhibited by FDX and OP-1118 but not significantly by vancomycin. Both FDX and OP-1118 (unlike vancomycin, rifaximin, and metronidazole) effectively inhibited sporulation by C. difficile. The inhibitory effect of FDX on C. difficile sporulation may contribute to its superior performance in sustaining clinical response and reducing recurrences and may also be beneficial in decreasing shedding and transmission of this pathogen.

Figure 1.

Development of total viable counts (filled symbols with solid lines) and heat-resistant spores (open symbols with dashed lines) in Clostridium difficile UK-14 strain: in the absence of drugs (•, ○), in the presence of fidaxomicin (FDX) (▴, Δ), in the presence of OP-1118 (▾, ▿), and in the presence of vancomycin (▪, □). The data represent the averages of 6 independent runs for no drug control, 4 independent runs for FDX, 2 independent runs for OP-1118, and 1 run for vancomycin. Error bars represent the standard error of the mean. Abbreviation: CFUs, colony-forming units.

Figure 2.

Development of total viable counts (filled symbols with solid lines) and heat-resistant spores (open symbols with dashed lines) in Clostridium difficile American Type Culture Collection 43255 strain: in the absence of drugs (•, ○), in the presence of fidaxomicin (FDX) (▴, Δ), in the presence of OP-1118 (▾, ▿), and in the presence of vancomycin (▪, □). The data represent the averages of 3 independent runs for FDX, 4 independent runs for vancomycin, and 4 independent runs for OP-1118. Error bars represent the standard error of the mean. Abbreviation: CFUs, colony-forming units.

Figure 3.

Development of total viable counts (filled symbols with solid lines) and heat-resistant spores (open symbols with dashed lines) in Clostridium difficile UK-14 strain: in the absence of drugs (•, ○), in the presence of metronidazole (▴, Δ), in the presence of rifaximin (▾, ▿), and in the presence of vancomycin (▪, □). The data represent averages of 6 independent runs for no control drug, 4 independent runs for metronidazole, 3 independent runs for rifaximin, and 1 run for vancomycin. The error bars represent the standard error of the mean. Abbreviation: CFUs, colony-forming units.