SMT19969 as a treatment for Clostridium difficile infection: an assessment of antimicrobial activity using conventional susceptibility testing and an in vitro gut model

Abstract

Objectives: We investigated the efficacy of the novel antimicrobial agent SMT19969 in treating simulated Clostridium difficile infection using an in vitro human gut model.

Methods: Concentrations of the predominant cultivable members of the indigenous gut microfloras and C. difficile (total and spore counts) were determined by viable counting. Cytotoxin titres were determined using cell cytotoxicity and expressed as log10 relative units (RU). Clindamycin was used to induce simulated C. difficile PCR ribotype 027 infection. Once high-level cytotoxin titres (≥ 4 RU) were observed, SMT19969 was instilled for 7 days. Two SMT19969 dosing regimens (31.25 and 62.5 mg/L four times daily) were evaluated simultaneously in separate experiments. MICs of SMT19969 were determined against 30 genotypically distinct C. difficile ribotypes.

Results: SMT19969 was 7- and 17-fold more active against C. difficile than metronidazole and vancomycin, respectively, against a panel of genotypically distinct isolates (P < 0.05). Both SMT19969 dosing regimens demonstrated little antimicrobial activity against indigenous gut microflora groups except clostridia. SMT19969 inhibited C. difficile growth and repressed C. difficile cytotoxin titres in the gut model.

Conclusions: These data suggest that SMT19969 is a narrow-spectrum and potent antimicrobial agent against C. difficile. Additional studies evaluating SMT19969 in other models of C. difficile infection are warranted, with human studies to place these gut model observations in context.

Keywords: Clostridium difficile; NAP1/027; antibiotics; cytotoxin; gut model; microflora.

Figure 1.

Time scheme in gut model experiments with SMT19969. CLI, clindamycin; CD, C. difficile spores; qds, four times daily.

Figure 2.

Mean viable counts (±SE) of obligate anaerobe bacterial groups (log₁₀ cfu/mL) in vessel 3 of the SMT19969 250 mg/L/day gut model (period E). TA, total anaerobes (obligate and facultative).

Figure 3.

Mean viable counts (±SE) of facultative anaerobe bacterial groups (log₁₀ cfu/mL) in vessel 3 of the SMT19969 250 mg/L/day gut model (period E). TFA, total facultative anaerobes.

Figure 4.

C. difficile total viable counts, spore counts (log₁₀ cfu/mL) and cytotoxin titres (RU) in vessel 3 of the gut model dosed with 250 mg/L/day SMT19969 (period E).

Figure 5.

C. difficile total viable counts, spore counts (log₁₀ cfu/mL) and cytotoxin titres (RU) in vessel 3 of the gut model dosed with 125 mg/L/day SMT19969 (period E).