The bacteria in your digestive system is so vital to health and well-being that it is sometimes called the ‘forgotten organ’
THE HUMAN GUT hosts a vast colony of microorganisms, mostly bacteria. This colony outnumbers the cells in a human body by a factor of 10, and its metabolic activities are so important that gut bacteria are sometimes called a “forgotten organ”. They perform a host of useful functions, including helping digestion, training the immune system, preventing the growth of harmful bacteria, producing vitamins for the body and regulating appetite. Modern interference with the composition of this gut flora may play a significant role in the sharp increase in obesity and autoimmune diseases, such as multiple sclerosis.
The average adult intestine is host to about 1.2 kg of bacteria. Bacteria account for up to 60 per cent of the dry mass of faeces, and an adult excretes his/her own weight in faecal bacteria every year. These cells and the genes they contain are known as the microbiome. The microbiome contains 3.3 million genes, dwarfing the human genome which contains only 20,000-25,000 genes. No two people have exactly the same microbiome, not even identical twins. Most people share a core complement of helpful bacterial genes, but can otherwise differ markedly in the number and types of microbiome bacteria. Since the DNA of all people is 99.9 per cent alike, individual health may have much to do with the genes in our microbiome.
Humans start out without a microbiome – the womb is normally bacteria-free – but we acquire one early in life. As the baby passes through the birth canal it picks up some of the mother’s bacteria, which then multiply. Breastfeeding, handling by adults, and interaction with siblings and the environment all contribute microbes so that, by late infancy, a complex microbiome has been established.
The microbiome aids digestion because it contains enzymes capable of breaking down complex plant carbohydrates, so the human host can extract nutrients from foods such as apples, potatoes and oats. Lab mice are available that have no microbiome because they were raised in a sterile environment. Such mice must be fed 30 per cent more calories than normal mice to achieve the same body weight because the bacteria salvage calories from otherwise indigestible plant fibre. Gut bacteria also synthesise vitamins such as vitamin K and vitamin B12 for the human host.
The stomach bacterium Helicobacter pylori was implicated in causing peptic ulcers in the 1980s and it is standard practice now to treat peptic ulcers with antibiotics to kill it. It has since been discovered that H pylori also regulates the stomach hormone ghrelin that causes feelings of hunger before mealtimes and whose levels decrease after a meal, causing your appetite to wane. If you have no H pylori, for example after antibiotic treatment, your ghrelin doesn’t decrease after eating and your appetite remains hearty.
Over 80 per cent of Americans hosted H pylori in past generations but less than 6 per cent of children now have this bacterium: they are now treated with repeated rounds of antibiotics that kill off H pylori and change the composition of the microbiome. This may be a factor in the rising levels of childhood obesity, causing difficulty in controlling appetite and possibly stimulating a class of body stem cells to turn into fat cells rather than into muscle or bone. Interestingly, altered composition of the microbiome has also been noted in autism.
Other work has shown that the microbiome trains our immune system to achieve a balance between attacking foreign invaders of the body and sparing normal body tissues. Artificially altering the microbiome could interfere with this and play a major role in the rise of autoimmune diseases such as Crohn’s disease, multiple sclerosis and type-one diabetes.
The public needs to learn to protect this “forgotten organ”. Why, for example, are we still abusing antibiotics when this produces so many ill effects? And we must stop our obsession with sterilising our environment.
Much excellent research on the microbiome is carried out at UCC in the Alimentary Pharmabiotic Centre. A good article on the microbiome by Jennifer Ackerman appears in the June edition of Scientific American.
William Reville is an emeritus professor of biochemistry and public awareness of science officer at UCC. understandingscience.ucc.ie