Continuing with my (apparent) theme of dissecting popular science, I’ve decided to talk about a rapidly growing field of study – that of the microbiome. For those of you who don’t know, the ‘microbiome’ refers to the diverse ecology of bacteria residing within our intestines. To put into context just how vast this range of microorganisms is: we have over 100 trillion tiny bacterial cells residing within our bodies (99% of which live in the gut), significantly more than we have human cells in the body. In the average adult, up to 2kg of bodyweight can be attributed to bacteria. This begs the question – why aren’t we all overcome with disease, considering how much bacteria we harbour?

The simple answer is that not all bacteria are bad. I’m sure that you’ll have seen adverts for yoghurt drinks claiming to contain lots of “friendly bacteria”. A significant rise in all probiotic sales (including tablets and powders) has been observed over the last five years, and further increases are expected within the future. Between 2015 and 2020, probiotic sales in Europe alone are predicted to increase by 15%. What do these “friendly bacteria” do, why are they so important, and why are they so marketable?

Let’s begin at the start of life – as we’re born, our skin is coated with the first bacteria that we’ve ever been exposed to (from our mother’s vaginal canal). They immediately begin to colonise us, fulfilling roles within the body such as helping us to digest breast milk, and kick-starting our immune systems. Certain species of gut bacteria have been shown to enhance the tolerance of our immune cells in the same way that anti-inflammatory steroids would, helping to reduce the risk of autoimmune diseases and allergies.

Birthing and breastfeeding promote the growth of two particular bacterial species – Lactobacillus, and Bifidobacteria. Children born via C-section take months to reach the expected diversity of gut bacteria (much longer than those of ‘natural birth’) due to lack of this initial exposure to maternal, vaginal bacteria. Babies born this way have an increased risk of developing allergies and asthma in later life, as their immune systems tend to overreact to perfectly normal environmental triggers. One study shows that by supplementing these babies with Lactobacillus at a young age, you can in fact reduce their risk of developing such allergies (whilst the same supplementation in natural births has no effect). Differences in gut bacteria aren’t permanent – any deviations seem to have levelled out by the age of seven. However, the allergies exhibited due to lack of immune system ‘training’ at birth continue to exist throughout life.

Another pivotal study highlighting the importance of the microbiome looks at the effects of raising mice in a perfectly sterile environment. From birth, the mice have absolutely no exposure to bacteria (not via their cages, their food or water, or their handlers). It was noted that the mice took a lot longer to digest food, most likely because they didn’t have the gut bacteria usually responsible for the breakdown of insoluble matter. They showed signs of shrunken digestive tracts, surrounded by much fewer blood vessels than would be expected. A reduced number of circulating immune cells was also observed (most likely because they’ve never been needed).

A recent area of interest concerning the microbiome is its relationship with the brain. A strong link between the gut and the brain has long been acknowledged (we have more nerves in our enteric nervous system than we do in our brain!), but it was only recently that a correlation between the occurrence of gastrointestinal conditions such as irritable bowel syndrome, and mental health problems such as anxiety and depression, was noted. In short, a significantly higher rate of mental health disorders is observed in those also diagnosed with gut problems. This could simply be a result of constant stomach pains generating a build-up of stress over time; however, in 2013 it was discovered that bacteria can produce signalling molecules that are able to make it past the obtrusive blood brain barrier (a series of tight blood vessels protecting our brain from chemicals in the blood). Such molecules include tyrosine and tryptophan – these are later converted into dopamine and serotonin, chemicals heavily implicated within our mood regulation. In this way, changes in our gut bacteria may potentially be of use in helping us to explain stress and low moods, as experienced within those suffering from anxiety and depression.

So, there could be a link between our mood and our microbiome. But what about our personalities? In a famous experiment performed several years ago, researchers used two different strains of laboratory mice (one known to exhibit extroverted behaviours, and one known to act in a more introverted manner), and extracted faecal matter (yes, poo) from each strain. The bacteria within these samples were extracted and ‘transplanted’ into the intestines of the other mouse. Lo and behold, the previously extroverted mice became shy and reserved, and those which were introverted began to exhibit outlandish and exaggerated behaviours. In other words – the two strains seemed to have swapped personalities! Although precisely how this came about is not yet understood, it is nonetheless a huge breakthrough in our understanding of how our gut influences our brain.

Lastly – and perhaps, more interestingly – it has been suggested that bacteria in our gut have the ability to influence our weight. One theory asserts that in the guts of obese people, the species of bacteria found are less diverse. More specifically, the prevailing species is one that aids in the breakdown of carbohydrates (enabling the individual to break down more food and absorb more calories per meal than they would have otherwise). Another theory states that bacterial signalling substances may cause inflammation in the gut and liver, slowing down metabolism and resulting in more energy being stored as fat. However, it should be noted that the whole “my slow metabolism is making me fat” urban myth is slowly being debunked (an argument that I’ll save for another time). A third theory is that because of the influence bacteria exhibit over our brains, they can change our diet by ‘requesting’ particular foods via chemical messengers – namely, ones full of energy and sugar that they can then use.

How much bacteria can actually affect our weight is still (very much) up for debate. However, in an experiment similar to the personality swap, previously sterile mice were administered harmless bacteria from the gut of an obese human. The mice quickly began to gain weight, at a much faster pace than mice that had received gut bacteria from a human of average weight. Why is this? While suggestions have been made, the mechanisms underlying these results are not yet entirely understood, largely due to the fact that this is such a new area of research.

I hope that this has provided at least a little insight into how underrated gut bacteria are. They play a large number of roles ranging from immune system development, to mood regulation, to weight gain (and loss!). They also have many other effects that we know about, as well as many others that we have yet to discover. A new and exciting field of study, we’ve only just begun to scratch the surface of our relationship with our microbiome. All of this leaves us with one critical question – are they just along for the ride, or are they an integral part of who we are?