Academic Reading — Test 36

Reading passage

The human digestive tract is home to an immense and varied community of microorganisms, collectively known as the gut microbiota. This population is dominated by bacteria, though it also includes viruses, fungi and other single-celled organisms. Estimates of the total number of microbial cells vary, but it is widely accepted that they roughly equal, and may even exceed, the number of human cells in the body. The genetic material carried by these microbes is so extensive that some scientists describe it as a second genome. Far from being passive residents, these organisms participate actively in many of the processes that keep their host alive and well, which is why the microbiota is sometimes characterised as a hidden organ. One of the most clearly understood functions of the gut microbiota relates to digestion. The human body produces enzymes capable of breaking down many foods, but it cannot process certain complex carbohydrates, particularly the dietary fibres found in fruits, vegetables and whole grains. When these fibres reach the large intestine largely intact, resident bacteria ferment them. This fermentation yields short-chain fatty acids, compounds that nourish the cells lining the colon and supply a modest amount of energy to the body as a whole. In addition, microbes manufacture several vitamins, including certain B vitamins and vitamin K, which the host would otherwise have to obtain entirely from food. The microbiota also assists in the absorption of minerals such as calcium and magnesium. The composition of an individual's microbiota is not fixed at birth but develops over the early years of life. Babies delivered vaginally acquire their first microbes from the mother during birth, whereas those born by caesarean section tend to be colonised by a somewhat different set of organisms. Breast milk further shapes the emerging community, partly because it contains sugars that the infant cannot digest but that selectively feed beneficial bacteria. By around the age of three, a child's microbiota begins to resemble that of an adult in its overall makeup, though it continues to shift in response to diet, illness, medication and environment throughout life. Diet exerts a particularly strong influence: a varied intake rich in plant matter tends to support a more diverse microbial community, and diversity is generally regarded by researchers as a marker of a healthy gut. The reach of the gut microbiota extends well beyond digestion. A substantial proportion of the body's immune cells are located in the tissues surrounding the intestine, and the microbes there help to train the immune system, teaching it to distinguish harmless substances from genuine threats. A balanced microbial community also occupies space and consumes nutrients that harmful bacteria would otherwise exploit, thereby providing a degree of protection against infection. Perhaps more surprisingly, scientists have identified communication between the gut and the brain, often referred to as the gut–brain axis. This two-way exchange takes place through nerves, hormones and immune signalling molecules, and emerging research suggests that the microbiota may influence mood and certain aspects of behaviour, although the precise mechanisms remain under investigation. When the balance of the microbiota is disturbed, a condition known as dysbiosis, health can suffer. Courses of antibiotics, while often necessary, do not target bacteria selectively and can sharply reduce microbial diversity, sometimes allowing opportunistic species to flourish. Dysbiosis has been linked with a range of disorders, including inflammatory bowel disease, obesity, type 2 diabetes and allergies, though it is frequently unclear whether the altered microbiota is a cause of these conditions or merely a consequence of them. Researchers are therefore cautious about claiming direct causation, and much current work aims to untangle this relationship. Interest in deliberately altering the microbiota for therapeutic benefit has grown considerably. Probiotics, which are live beneficial bacteria taken in foods or supplements, and prebiotics, which are dietary fibres that feed existing helpful microbes, are both widely marketed, though the evidence for many specific products remains mixed. A more dramatic intervention is faecal microbiota transplantation, in which stool from a healthy donor is introduced into a patient's intestine. This procedure has proved strikingly effective against persistent infections caused by the bacterium Clostridioides difficile, restoring a healthy community where antibiotics have failed. Scientists caution, however, that the microbiota is highly individual, and a treatment that benefits one person may have little effect on another. As understanding deepens, many anticipate that future medicine will increasingly take a person's microbial inhabitants into account.