Introduction

Many of our native plant medicines have the ability to help in the management of conditions affecting the gastrointestinal tract⁽¹⁾. While these have diverse actions and undoubtedly act on many levels concomitantly to prevent and treat such conditions, their direct effects on the gut microbiome, are likely to be contributory.

The gut microbiome is an intricate and dynamic ecosystem of microbes that is strongly influenced by our environment, diet and genes, and altering it can significantly influence our predisposition to and outcomes from many illnesses. While these extend way beyond gastrointestinal conditions alone^(2,3), the association between modulation of the gut microbiome and digestive health, is of particular interest.

Furthermore, it is these intestinal microbiota that play an important role in both the activation and metabolism of medicinal plant phytochemicals themselves, just as they break down proteins, carbohydrates and fats in foods, to simpler and bioavailable molecules. A diverse and healthy microbiome is thus conducive to ensuring optimal processing and bioavailability of medicinally active phytochemicals, and thus achieving favourable clinical outcomes in phytotherapy.

Probiotics (live microorganisms that when administered in adequate amounts confer a health benefit) are now highly regarded, but we’re also increasingly learning about the important role of prebiotics, in feeding and selectively promoting probiotic growth and human health. These include plant foods such as psyllium, onion, asparagus, garlic, kumara, chicory, Jerusalem artichoke, bananas, beetroot and oats, which provide complex carbohydrates such as inulin and other beneficial fibres to support a healthy gut microbiota and immune system.  Additionally, there seem to be positive contributions made by other specific primary or secondary components of many plants and mushrooms, to the microbiome community and subsequently to health outcomes.

Native plants and the microbiome

Regular ingestion of various herbal teas including green, kuding and baical skullcap teas can exert beneficial effects on the gut microbiota, including an increased number of unique bacterial genera, and changes in the relative abundances of particular species^(4-6). Catechins and insoluble fibre contribute to these benefits, and exert a prebiotic-like effect on gut microbiota⁽⁷⁾. Supplementation with certain tannins has also been found to increase the diversity and abundance of butyrate-producing and probiotic bacteria and metabolism of amino acids⁽⁸⁾. In fact the antioxidant and disease preventative properties of many polyphenolic rich plants, may correlate to some extent with their effects on gut bacterial species⁽⁹⁾.

While research is required, many native plants from Aotearoa New Zealand are very rich in a diverse range of polyphenolic compounds and are likely to also produce similar benefits. While tannins and other larger ones of these have poor oral bioavailability, they can nevertheless facilitate significant changes in the abundance of bacterial species associated with a wide range of health outcomes. The hydrolysis of tannins and flavonoids by gut microbiota is also being increasingly recognised as producing smaller, bioavailable and bioactive metabolites, with many health benefits^{(10, 11)}.

Controlling infections

Antibiotics are widely used to eradicate unwanted bacteria, yet they are generally not very selective, and often produce serious negative impacts on beneficial bacteria and the gut microbiome⁽¹²⁾. Plants with a narrower spectrum of antimicrobial activity and when carefully combined, have the ability to provide a more targeted action, and thus reduced adverse events⁽¹³⁾.   They should therefore be used more as first line treatments for many types of infection, and more research into this as well as their combined use with antibiotics, is urgently needed.

The microbiota is also involved in reducing and preventing colonization by enteric pathogens through the process of competitive exclusion and the production of antimicrobial substances. Plants that enhance this production of bacteriostatic and bactericidal substances by the microbiota, are therefore potential interventions to deal with some pathological infections⁽¹⁴⁾.

Mānuka (Leptospermum scoparium), Kānuka (Kunzea ericoides), Harakeke (Phormium tenax), Horopito (Pseudowintera colorata), Tanekaha (Phyllocladus trichomanoides), Rewarewa (Knightia excelsa) Totara (Podocarpus totara, Rimu (Dacrydium cupressinum) and Pukatea (Laurelia novae-zelandiae), all exhibit good activity against specific pathogens. 

Animal studies are increasingly showing the microbiome to have a positive impact not only on gut physiology, but also the immune system of the host. Part of the mechanism of Echinacea purpurea’s immunomodulatory effects may involve its influences on the gut microbiota, as shown by improvement in immunosuppressed ducks in conjunction with increased abundance of beneficial intestinal bacteria⁽¹⁵⁾.

Antibacterial and antifungal actions are prominent activities of many native plants, and traditional uses for gastrointestinal tract infections, were once common. Tannin rich native species such as Mānuka can treat dysentery and diarrhoea, and most tannins also have intrinsic antibacterial properties. The powerful astringent and antimicrobial effects produced by infusions and hydroethanolic liquid extracts of Mānuka’s leaves and stems, make it a useful component of a treatment for gastrointestinal infections.

In its living state, Mānuka itself has around 200 different bacteria within its leaves, stems and roots, that make up a rich community of endophytes (microorganisms that live within and near plants, in a symbiotic relationship). Several of these bacteria themselves produce antifungal or antibacterial compounds as part of their competitive survival⁽¹⁶⁾. Ingestion of preparations made from its leaves and stems will deliver a range of phytochemicals with the potential to modulate the gut microbiome, and thus provide digestive system benefits. The often symbiotic relationships between endophytes and plants, and those between microbes and humans, appear to have much in common.

Koromiko (Hebe stricta) is another effective remedy used for diarrhoea and dysentery among Māori and European settlers, including by New Zealand soldiers during the Second World War. Rebalancing of different microbial species within the gut, is probably involved in its antidiarrhoeal and anti-inflammatory properties.

Intestinal mucosa tonics

Damage to the integrity of the intestinal biofilm can result in alteration in intestinal permeability, a condition widely described asleaky gut. When this occurs, the immune system can become weak or dysregulated, making conditions such as irritable bowel syndrome or inflammatory bowel disease more likely⁽¹⁷⁾. Ulcerative colitis, Crohns and irritable bowel syndrome are all associated with enhanced secretion of proinflammatory cytokines, poor maintenance of the epithelial barrier, and gut dysbiosis^{(18, 19)}.

Kawakawa (Piper excelsum) is a plant widely used for gastrointestinal inflammation and has many pronounced digestive system benefits^{(20, 21)}. As with other Piperaceae species, influences on gastrointestinal absorption and gut permeability, and vascular barrier protective effects have been shown for Kawakawa and some of its amide constituents, including pellitorine, piperine and piperdardine^{(22, 23)}.  Lignans are also prominent constituents, and given other lignans are known to interact with gut microbiota, microbiomal modulations through these phytochemicals within Kawakawa, may also take place.

Other plants with broad-ranging effects on the digestive system, are Horopito (Pseudowintera colorata) and Akeake (Dodonaea viscosa). Both plants exhibit astringent, anti-inflammatory and some antimicrobial properties which probably impact population levels of some microbiome species in a negative manner, and others in a potentially facilitatory manner. An Indian variety of Dodonaea viscosa has been shown to be gastroprotective in animal studies⁽²⁴⁾, suggesting a potentially useful role in the management of peptic ulcers⁽²⁵⁾.

Polysaccharides

These are another type of secondary metabolite whose oral bioavailability is limited, but are increasingly being recognized for the complex interactions they have with microbes within the gut. These include modulation of the gut microbiota composition itself, metabolism of polysaccharides to short chain fatty acids, and polysaccharide-induced modulation of the production of gut microbiota metabolites such as trimethylamine, tryptophan and lipopolysaccharides⁽²⁶⁾. The widely used Psyllium husk (Plantago ovata), increases faecal water content and improves constipation while increasing populations of butyrate-producing microbiota⁽²⁷⁾. Amelioration of antibiotic-associated diarrhoea has been associated with  a prebiotic effect of polysaccharides from the medicinal fungus Hoelen (Poria cocos)⁽²⁸⁾.

Hoheria (Hoheria populnea), is a native tree whose various different organs are all rich in polysaccharides.  Like Slippery Elm and other polysaccharide hydrocolloid rich plants, Hoheria can help reduce symptoms of dyspepsia or gastritis when taken either as an infusion or hydroethanolic liquid extract. As with many other polysaccharide-rich plants or medicinal fungi, beneficial effects on the gut microbiome, also seem likely.

Bitters

Medical herbalists are generally taught that it is the stimulatory effects of bitter-tasting plants on every component of the gastrointestinal system, which accounts for their many beneficial and tonic like actions on digestive processes and thus overall health. We now also know that activation of bitter receptors is involved in the regulation of appetite, insulin sensitivity, airway innate immunity, and other physiological processes. The ability of bitter tasting plants and their inclusion in the diet or regular herbal tonics to positively influence the gut microbiome community, is being increasingly revealed^(29-31).

Native plants such as Kohekohe (Dysoxylum spectabile) and Tanekaha make excellent bitter substitutes to classical European bitters such as Gentian and Wormwood.  These are also likely to have complex but beneficial actions on the gut microbiome.

Summary

Humans have a wide variation in the makeup of their gut microbiome.  This potentially results in variations in microbial metabolism of many phytochemicals, and thus the resulting therapeutic activities of medicinal plants. The relationships between plants and microbes works in both directions though, and there are a multitude of ways in which the ingestion of many plants can produce health benefits through influencing the functions of these bacteria found without our gastrointestinal tract.

While we are only just starting to understand elements of the complex human gut microbiome, it seems certain to provide innovative targets for the incorporation of plants native to Aotearoa New Zealand, to help prevent and treat several gastrointestinal conditions.

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