What Is the Microbiome? The Organisms That Outnumber Your Own Cells

For most of the history of microbiology, studying a bacterium meant culturing it, and the great majority of gut organisms will not grow outside the gut, so they were invisible. The field was opened by sequencing, first by amplifying a particular ribosomal gene that differs enough between species to act as a barcode, and later by shotgun sequencing all the DNA in a sample, which identifies organisms and reveals what genes they carry. The picture that emerged is of a dense community, concentrated overwhelmingly in the colon, dominated by two bacterial groups with substantial representation from several others, and differing enormously between individuals: two healthy people may share only a minority of their species while having similar overall functions covered, which is why the field increasingly talks about what the community does rather than who is in it. The often-repeated claim that bacteria outnumber human cells ten to one was traced to a back-of-envelope estimate from 1972 and revised in 2016 to roughly one to one.

Several functions are well established and not controversial:

• •Fermenting dietary fibre that human enzymes cannot break down, producing short-chain fatty acids including butyrate, which is the main energy source for the cells lining the colon and has anti-inflammatory effects
• •Synthesising vitamin K and several B vitamins
• •Occupying niches and producing compounds that make it harder for pathogens to establish, which is why antibiotic treatment can be followed by an opportunistic infection
• •Training and calibrating the immune system, particularly in early life, with germ-free animals showing profoundly abnormal immune development
• •Metabolising drugs and other compounds, altering the effectiveness of some medicines, including certain cancer immunotherapies
• •Maintaining the gut barrier, since butyrate supports the tight junctions that keep intestinal contents where they belong

The strongest evidence for treating the microbiome as a therapeutic target comes from a single disease. Clostridioides difficile causes a severe, often recurrent colitis that typically follows antibiotic treatment, and repeated courses of further antibiotics frequently fail. Faecal microbiota transplantation, transferring processed stool from a screened donor into a patient's gut, cures a large majority of recurrent cases, with a landmark randomised trial in 2013 stopped early because the results were so much better than the antibiotic comparison that continuing was judged unethical. Regulators have since approved standardised microbial products for this indication. The success is instructive precisely because it is narrow: it works when the resident community has been destroyed and needs replacing wholesale, and attempts to apply the same procedure to inflammatory bowel disease, obesity, autism and metabolic conditions have produced far weaker and less consistent results.

A genuine communication system links the two, running along the vagus nerve, through immune signalling and through metabolites that enter the bloodstream, and gut bacteria produce or influence the production of neurotransmitters including serotonin, most of which is made in the gut although it does not cross into the brain. Animal experiments have shown striking effects: germ-free mice are behaviourally abnormal, and transferring gut communities between mouse strains can transfer aspects of anxiety-like behaviour. In humans the evidence is much weaker. Differences in gut communities have been observed in depression, anxiety and Parkinson's disease, and observation cannot separate cause from consequence, since mood, medication and diet all change what lives in the gut. Trials of probiotics for mood have produced small and inconsistent effects. The reasonable position is that the axis exists, that it matters, and that no specific psychiatric intervention based on it is yet supported.

The commercial enthusiasm has run far ahead of the science, and a few rules of thumb help. Most published findings are associations from observational studies, which cannot establish direction, and the field has a documented problem with small samples and analyses that are difficult to reproduce. Probiotic supplements contain a small number of species, usually of two genera chosen because they are easy to manufacture, which may not survive passage, may not colonise, and have specific evidence for specific conditions rather than general benefit; a 2018 study found supplementation after antibiotics actually delayed recovery of the native community compared with doing nothing. Home testing kits report which organisms are present without any established reference range for what a good result looks like. What is well supported is unglamorous: a diet high in varied plant fibre reliably increases microbial diversity and short-chain fatty acid production, fermented foods have shown measurable effects in controlled feeding studies, and avoiding unnecessary antibiotics protects a community that takes months to recover from each course.

The human microbiome is a community of roughly thirty-eight trillion microbial cells, concentrated in the colon, identified by sequencing because most cannot be cultured, and carrying vastly more genes than the human genome. It ferments fibre into short-chain fatty acids, makes vitamins, resists pathogens, trains the immune system and metabolises drugs. Faecal transplantation cures recurrent Clostridioides difficile infection and has not translated to other conditions, the gut-brain axis is real but has not yet produced a supported treatment, and dietary fibre remains the best-evidenced intervention.