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Manuka Honey Inhibition of C. Diff Growth is Inconsistent

New data shows that clinical isolates of C. difficile are variably affected by manuka honey in terms of growth inhibition and bactericidal activity.

According to a new study, Manuka honey has potential to inhibit proliferation of Clostridioides difficile (C. difficile) spores, yet its universality in all situations as well as ability to completely eradicate spores is limited.

In previous studies, manuka honey has been shown to contain phytochemicals, which promote antimicrobial activity. Furthermore, these studies have uncovered an antimicrobial effect against C. difficile.

Investigators at the Keck School of Medicine assessed the antimicrobial activity against clinical C. difficile isolates as well as its effects on spore count. Their aim was to further characterize the honey’s in vitro antibacterial effect on the toxigenic spores.

To do this, they acquired 20 clinical isolates cultivated from diarrheal fecal specimens. Of the total, 17 were toxigenic. They then vortexed equal volumes of the specimens and incubated them at room temperature.

Using the broth dilution method, the team determined the minimum inhibitory concentrations (MIC) of 4 different methylglyoxal (MGO) grades (30+, 100+, 250+, and 400+) for manuka honey. They controlled for growth and sterility, and all MIC experiments were performed in duplicate.

To determine whether there were any significant differences in MIC values between MGO grades, they used the Kruskal-Wallis test.

Minimal bactericidal concentrations were determined using the same dilution method. They obtained baseline counts from the grown control well and 10 µL from each well showing no growth was plated to BHI agar.

In order to assess sporicidal activities, they selected a C. difficile strain with an MIC of 6% and MBC of 30% in MGO 400+ honey and enumerated total viable cell and spore counts from 0-96 hours.

Thus, they determined that the MICs of all isolates ranged from 4%-30%. The MIC in the 90th percentile was 22% for MGO 30+ and 100+ honeys, 18% for MGO 250+, and 14% for MGO 400+.

For MICs in the 50th percentile, values were at 10-14% for all honey grades. Further, 2 isolates yielded MICs of >30% for all honeys tested.

Additionally, MGO grades did not appear to determine or correlate with differences in MIC values (P = .57).

In terms of observed bactericidal activity, the investigators noted a variability depending on the grade of honey and the clinical isolate. About ½ of the isolates demonstrated some measurable bactericidal activity.

Thus, the MBC/MIC ratio was ≤4. In the remainder of the isolates, the bactericidal activity was beyond the upper limit of 30%

Growth kinetics in honey also showed total viable cell counts remaining >105 colony-forming units (CFU)/mL. Spore counts remained within 1-log of baseline in honey but steadily increased to >105 CFU/mL in the drug-free control after 96 hours.

“Although we did not find manuka honey to be universally bactericidal or inhibitory against C. difficile growth, its global effect on the gut microbiome and the host inflammatory response will be important additional considerations for potential use in CDI [Clostridioides difficile infection],” the investigators wrote.

Further, they acknowledged that the in vitro effects of manuka honey with the in vivo impact in CDI is a domain for further exploration.

The study, “The Bactericidal Activity and Spore Inhibition Effect of Manuka Honey against Clostridioides Difficile,” was published online in MDPI Antibiotics.