Your Intestinal Tract: It’s a jungle in there!
There’s an ecosystem teeming with undiscovered species, and learning how to manage it could yield cures for human diseases. Daphne Chung, New Scientist vol 182 issue 2444 – 24 April 2004, page 42
It is hardly a glamorous job, but Gibson and other researchers are convinced that studying faeces, or rather the microbes they contain, will offer a radical approach to improving our health.
Their belief stems from accumulating evidence that gut bacteria play a pivotal role in a number of chronic and sometimes fatal diseases. These include inflammatory bowel diseases such as ulcerative colitis, irritable bowel syndrome, a painful condition afflicting more than a fifth of people in developed countries, and cancers such as colon cancer. Gut bacteria may even help shape our immune system and susceptibility to allergic conditions such as eczema.
The plan is to manipulate the behaviour of these bacteria in our favour. Beneficial bacteria could be encouraged to grow and crowd out or kill the harmful ones – a form of biological pest control for the gut. Researchers hope such treatments could tackle diseases that are currently hard to treat. What’s more, encouraging the growth of harmless bacteria we all have in our guts anyway is unlikely to cause any nasty side-effects. But first, we need to understand more about gut bacteria, and this is an enormous challenge.
Your gut is teeming with microbes. It is home to several hundred species of bacteria, along with viruses and other organisms such as yeasts. There are about a trillion bacteria per gram of stool passing through your large intestine, and they make up some 60 per cent of the solid mass of faeces. In fact, there are so many bacterial cells in your gut that they outnumber the cells of your own body by at least a factor of 10.
The interactions between all these bacteria are so complex that researchers view the community as an ecosystem. Bacteria compete with each other for space and food, and some even try to bump off their rivals. Two of the key genera, Lactobacillus (also known as lactobacilli or lactic acid bacteria) and Bifidobacterium (bifidobacteria), make acidic chemicals such as lactic acid that lower the pH of the gut and kill some other species. Other groups include the methane-producing methanogens and the sulphate-reducing bacteria, which make hydrogen sulphide, and each prefers to live in different parts of the gut (see Diagram). Some species make proteins called bacteriocins that kill other bacteria.
It is only in the past decade that biologists have been able to appreciate just how big and complex this ecosystem is. Before then, they had to grow bacteria in the lab before they could study them. Many species of bacteria, however, do not grow in the lab, meaning they were overlooked. In the 1990s, molecular biologists developed a way of extracting and copying bacterial genes from samples without first having to grow the bacteria in the lab. This meant they could directly test, say, seawater or soil for DNA that would reveal the existence of unknown species.
When Gibson and his team applied these techniques to the human gut in 1999, they discovered that only a quarter of the DNA they found corresponded to organisms whose sequences were recorded in gene databases (Applied Environmental Microbiology, vol 65, p 4799). This meant that a staggering three-quarters of the bacterial species in our guts were (and largely still are) unknown. So researchers hoping to understand the relationship between our guts and their bacteria really have their work cut out.
Such is the scale of the problem that 64 research groups in Europe have teamed up to form the EU Human Gut Flora Project, which aims to determine the links between food, intestinal bacteria and human health and disease. “It’s an unexplained area,” says Gibson, “but with a huge capacity for causing discomfort. More people go to the doctor with gut complaints than anything else.”
There is already strong evidence that gut bacteria can produce harmful chemicals. Bacteria in the gut live on components of the food we eat, as well as on substances that our guts produce naturally, such as digestive juices. It is perhaps not surprising, then, that researchers have long suspected that gut bacteria can make us ill by producing harmful metabolic by-products, perhaps chemicals that cause inflammation by damaging or irritating gut cells, or others that damage DNA or encourage tumours to grow, resulting in colon cancer…