The Gut Microbiome: What Practitioners Need to Know

20 years ago the gut microbiome was a specialist research topic. Today it is discussed in clinics, news media and supplement advertising as a key to weight, mood, immunity and chronic disease. The science has advanced remarkably, and some of what has been learned has direct clinical value. But the distance between what researchers have discovered and what practitioners can reliably do with that knowledge remains substantial. This article aims to map both.

What the microbiome is

The human gut hosts a community of bacteria, archaea, fungi and viruses, numbering in the tens of trillions of cells, concentrated in the colon. Hundreds of bacterial species can be found in a single person, and collectively their genes greatly outnumber human genes. The dominant bacterial groups in most adults belong to a small number of phyla, notably Bacillota, formerly Firmicutes, and Bacteroidota, formerly Bacteroidetes, but the particular species and strains differ widely from person to person.

This variation is one of the most important and least appreciated findings. There is no single healthy microbiome. Healthy people in different countries, on different diets and at different ages have very different microbial communities. Features associated with health tend to be functional, such as diversity and the capacity to ferment fibre, rather than the presence of particular species.

How the microbiome develops

Colonisation begins at birth. Mode of delivery, breastfeeding or formula feeding, early antibiotic exposure and the family environment shape the infant microbiome, which matures towards an adult like community over the first few years of life. Early life disruptions have been associated with later allergic, metabolic and inflammatory conditions, although distinguishing causation from association is difficult. In adulthood the microbiome is relatively stable but responsive, changing with diet, medicines, illness, travel and age.

What the microbiome does

• Fermentation. Colonic bacteria ferment dietary fibre into short chain fatty acids, which nourish colonic cells, support the intestinal barrier and influence immune and metabolic signalling.
• Immune education. Microbes and their products train the immune system, promoting tolerance to harmless antigens and appropriate responses to pathogens.
• Colonisation resistance. A healthy community helps prevent pathogens from establishing themselves.
• Metabolism of bile acids, drugs and dietary compounds. Microbes transform bile acids, activate or inactivate some medicines, and convert plant compounds into forms the body can use.
• Vitamin synthesis. Some bacteria produce vitamin K and certain B vitamins.
• Signalling to distant organs. Through metabolites, immune signalling and the vagus nerve, the microbiome communicates with the liver, brain and other organs.

What changes the microbiome

Diet

A human study showed that switching between plant based and animal based diets altered microbial composition and function within days. Fibre intake is a major determinant: fibre rich diets support fermenting bacteria, while diets low in fermentable fibre are associated with reduced diversity. A randomised study found that increasing fermented food intake increased microbial diversity. Observational data from a large citizen science project suggested that people eating a greater variety of plants had more diverse microbiomes.

Antibiotics

Antibiotics can reduce diversity substantially, and while communities usually recover partially within weeks, some species may take months to return or may be lost. Repeated courses have cumulative effects.

Other medicines

A systematic screen of more than a thousand marketed drugs found that around a quarter of non antibiotic drugs inhibited the growth of at least one gut bacterial species in the laboratory. Proton pump inhibitors, metformin, laxatives and some psychiatric medicines are among those with documented effects in humans. Some of metformin’s benefits may be mediated partly through the microbiome.

Other factors

Age, illness, travel, stress, sleep, alcohol and physical activity have all been associated with changes, although effect sizes vary.

The microbiome and disease

Differences in microbial composition have been reported in inflammatory bowel disease, irritable bowel syndrome, obesity, type 2 diabetes, fatty liver disease, colorectal cancer, allergic and autoimmune diseases, depression and neurological conditions. Establishing whether these differences cause disease, result from it, or simply reflect associated factors such as diet and medicines is the central challenge.

Animal studies provide some evidence of causation: transferring microbiota from obese to germ free mice, for example, transferred increased fat gain. Human evidence of causation is strongest for Clostridioides difficile infection, where faecal microbiota transplantation, transferring stool from a healthy donor, achieves high cure rates for recurrent infection and outperformed antibiotics in a landmark trial. For most other conditions, microbiome based treatments remain experimental.

What about dysbiosis?

Dysbiosis is used to describe an imbalanced or disrupted microbiome. The concept is useful when it describes clear disruptions, such as after antibiotics or in C. difficile infection. It becomes less useful when applied to minor compositional differences, because there is no agreed definition of a normal microbiome against which to measure imbalance. Practitioners should be cautious about diagnosing dysbiosis from a stool microbiome profile alone.

Testing

Commercial stool microbiome tests use sequencing to report which microbes are present and in what proportions, often with interpretations and recommendations. Current limitations are significant: results vary between laboratories and methods, composition fluctuates day to day, reference ranges for a healthy microbiome are not established, and there is little evidence that acting on results improves outcomes. Validated stool tests for specific purposes, such as pathogen detection, C. difficile testing and faecal calprotectin, are different and remain clinically useful.

Probiotics

Probiotics are live microorganisms that, in adequate amounts, confer a health benefit. Their effects are strain specific: evidence for one strain does not apply to another. Gastroenterology guidance has concluded that for most digestive conditions evidence is insufficient to recommend probiotics routinely, with exceptions such as certain strains for preventing C. difficile infection in adults taking antibiotics and for preventing necrotising enterocolitis in preterm infants. Probiotics usually do not permanently colonise the gut, and in immunocompromised or critically ill patients they carry a small risk of infection.

What practitioners can do now

1. Encourage dietary diversity and fibre, particularly a wide variety of plant foods.
2. Include fermented foods where tolerated and appropriate.
3. Support prudent antibiotic use, recognising their value when needed and their costs when not.
4. Review medicines, particularly long term proton pump inhibitors, for ongoing need.
5. Recommend probiotics selectively, choosing specific strains for indications with evidence.
6. Use validated stool tests for defined questions, and treat commercial microbiome profiles as research information rather than diagnosis.
7. Address other mediators of gut health, including sleep, stress and alcohol.

An illustrative case

A 40 year old woman brings a commercial microbiome report stating that she has low diversity and dysbiosis, with a list of recommended supplements costing a substantial sum each month. She has occasional bloating but otherwise feels well. Her diet is limited in variety, and she has taken 3 courses of antibiotics in the past year for recurrent sinus infections.

The practitioner explains what the test can and cannot show. The plan focuses on increasing the variety of plant foods she eats each week, adding fermented foods she enjoys, and working with her doctor on the sinus problem to reduce the need for further antibiotics. The supplements are not started. Her bloating improves as her diet broadens gradually. The report prompted a useful conversation, but the plan rested on what is known rather than what was sold.

The frontier and the clinic

Microbiome science is one of the most exciting fields in biology, and it is likely to transform some areas of medicine. For now, the most reliable ways to support a healthy microbiome are familiar ones: a diverse, fibre rich diet, fermented foods, careful use of antibiotics and attention to overall health. Practitioners who hold the frontier and the clinic in their proper places can engage patients’ curiosity without overpromising.