Abstract
Clostridioides difficile colitis overgrowth occurs when the normal gut microbiome becomes disrupted, often due to antibiotics. Effective treatment remains elusive, due partly to the persistence of its spores in the gut. Natural substances like manuka honey offer an alternative antimicrobial mechanism of action to conventional antibiotics. We investigated the antibiotic activity of manuka honey against 20 C. difficile isolates. The minimum inhibitory concentrations (MICs) and minimal bactericidal concentrations (MBC) of manuka honeys of methylglyoxal (MGO) grades 30+, 100+, 250+, and 400+ were determined based on broth microdilution. Sporicidal activity was assessed in a range of honey concentrations by enumerating total viable cell and spore counts at 0–96 h after organism inoculation. The MICs of C. difficile ranged from 4% to >30% (w/v). MIC50 for the four MGO grades were similar at 10–14%. MBC results for the majority of isolates were distributed bimodally at MBC/MIC ratios ≤4 or MBC >30%. Growth kinetics in honey showed total viable cell counts remaining >105 colony-forming units (CFU)/mL at all time points, whereas spore counts remained within 1-log of baseline (102 CFU/mL) in honey but steadily increased in the drug-free control to >105 CFU/mL by 96 h. Manuka honey demonstrated variable inhibitory and bactericidal activity against C. difficile. MGO grade had no noticeable impact on overall MIC distributions or bactericidal activity. Although manuka honey could inhibit spore proliferation, it did not eradicate spores completely.
Highlights
Clostridioides difficile is a major cause of antibiotic-associated diarrhea worldwide, accounting for 20% of cases among hospitalized patients [1]
MIC results at the 50th percentile (MIC50) values were similar at 10–14% for all honey grades (Table 1, Figure 1)
There were no significant differences in minimum inhibitory concentrations (MICs) values between MGO grades (p = 0.57, Kruskal–Wallis test)
Summary
Clostridioides difficile is a major cause of antibiotic-associated diarrhea worldwide, accounting for 20% of cases among hospitalized patients [1]. The incidence of infections varies with geographic region, with a low incidence reported in Asia, whereas, in the U.S, C. difficile is the most common pathogen causing healthcare-associated infection [2,3]. This spore-forming Gram-positive bacterium can colonize the gastrointestinal tract and cause a toxin-mediated disease when the gut microbiome is disrupted, most commonly from antibiotic therapy. Modulation of the host response to prevent CDI or recurrence such as through vaccination remains investigative at this time [6]. Standard therapy for C. difficile infection (CDI) includes vancomycin or metronidazole, which, while active against vegetative organisms, do not effectively inhibit spore formation [7]. Investigations of compounds capable of killing spores have expanded to biological compounds such as lauric acid, Antibiotics 2020, 9, 684; doi:10.3390/antibiotics9100684 www.mdpi.com/journal/antibiotics
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