In vitro study of the interaction between human gut microbiota and herbal extracts using willow bark extract as an example

The human gut is inhabited by a high number and variety of microorganisms which influence the host's health and are involved in the metabolism of many ingested compounds [1,2,3]. Due to their low bioavailability, many plant constituents reach the distal gut, where they can affect the microbiota, and can be metabolized to smaller, better bioavailable, and bioactive compounds [3]. Willow bark extract (WBE), commonly used against pain and inflammation, contains salicin derivatives known to be metabolized to salicylic acid by intestinal bacteria and liver enzymes. However, salicylic acid serum levels are too low to fully explain the extract's activity, and other constituents may also be involved [4,5]. In the current in vitro study we investigated the metabolism of ethanol 70% WBE by human gut microbiota, and its impact on diversity and viability of gut microbiota. 10% human faecal suspension was incubated with WBE at two concentrations (2 and 10 mg/ml) under physiological conditions (anoxic, 37 °C). Samples were taken at 0, 4 and 24h. Changes in the metabolic profile of the incubation mixture were analysed by UHPLC-HRMS. The microbiome composition was analysed by next generation sequencing of DNA extracted conventionally and of DNA from propidium monoazide-pretreated samples in order to mask the background DNA of dead cells.The main constituents identified at t₀ were phenolic compounds like salicin derivatives, naringenin glycosides, and procyanidins. They were readily metabolized by the gut bacteria to small phenolic metabolites like hydroxyphenylpropionic acids, hydroxyphenylvaleric acids, and salicylic acid. Interestingly, the applied extract dose had a strong impact on the occurrence and speed of metabolic reactions, but also on the composition of the microbiome. Along with the chemical compounds, a number of microorganisms increased significantly, like different Clostridiales (Faecalibacterium; PMA-treated sample, WBE 2 mg/ml), or Parabacteroides (WBE 10 mg/ml).

Acknowledgements: We thank Bionorica (Neumarkt, Germany) for providing us with the willow bark extract. We thank K. Jensen (NAWI Graz Central Lab "Environmental, Plant & Microbial Metabolomics") for support with the LC-HRMS instrument.

Keywords: Willow bark, salicin, gut microbiota, metabolization.

References:

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[5] Nahrstedt A, Schmidt M, Jäggi R, Metz J, Khayyal, MT. Willow bark extract: The contribution of polyphenols to the overall effect. Wien Med Wochenschr 2007; 157: 348 – 351

No conflict of interest has been declared by the author(s).