The hidden world inside your gut: the microbiome

Peer into the microscopic world that inhabits the human body

By Elin Bevan

When we think of other worlds, distant planets or alien civilisations often immediately spring to mind. However, there is a huge, concealed world right in front of us – in fact, inside us! Over 70% of our body is not human; it is made up of microorganisms living in a variety of habitats within us. They are collectively called the human microbiome.

Microbiomes show great diversity, varying hugely from one person to another; differing as much as 80–90% between individuals. There are estimated to be 10–100 trillion microbial cells living on or in each person and, genetically, they have a staggering total number of different genes. The human genome encodes 20,000 genes altogether – whether they are expressed or not – but the gut microbiome alone encodes 3,300,000 non-redundant genes. When considering their miniscule size, this is even more astounding.

Our bodies are home to many, varying microbiomes (also called microbiota); on the skin, in the mouth, and – arguably most importantly – in the gut. A difference in the constituents making up the oral and faecal microbiomes was seen as early as the 1680s, and the term was first formally introduced in 2001. In the years since then, it has become a huge research focus. The gut microbiome is sometimes called the “forgotten organ” of the human body. This indicates how important it is for human health! For example, it has been linked to obesity, gastrointestinal disorders such as IBS (irritable bowel syndrome), and even mental health issues such as anxiety.

So how does this microscopic colonisation arise from a sterile foetus in the womb? Microbes are acquired from the external environment, and the first way this happens is through childbirth. Studies have shown that as soon as 20 minutes after birth, babies start to develop their own microbiome and, interestingly, this differs depending on the mode of delivery; if birthed vaginally, their microbiome resembles that of the mother’s vagina, but if delivered by caesarean section, babies have a skin-like microbiome.

Gut microbiota play a huge role in digestion and nutrition. The microbes present in the gastrointestinal tract help break down food (hydrolysing it) and, as the microbiota vary a lot between individuals, this can have differing effects. Some microbes are more efficient at hydrolysis, so they can release more energy from food, which leads to obesity in some people. Evidence to support this has come from research carried out in mice; when an ‘obese microbiome’ was transplanted into mice, they put on more weight than control mice, despite being given the same food.

A similar experiment looked at IBS and anxiety. When faecal transplants were put into mice from patients with these symptoms, they developed intestinal dysfunction and anxiety-like symptoms, respectively. This is a huge revelation, as it shows the gut-brain-axis goes both ways; we knew things like stress could upset the gastrointestinal system, but now we understand that this can also happen in reverse.

These studies, as well as other research into human microbiota, show that the factors influencing susceptibility to certain illnesses are not just confined to genetics and lifestyle, as previously thought; the microbiome has a huge influence. This hidden world is key to our health and now that we have discovered it, we must learn all we can from it.

From Issue 18