They form a vital part of your biology, digesting your food, interacting with your immune system and even
affecting your mood. Just as different plants and animals live in different habitats on Earth, different species of microbe colonise different parts of your body. The skin on your arms and legs is dry and the temperature is unpredictable, like a desert, so few species are hardy enough to make it their home. But the lining of your gut is warm, wet and full of nutrients, like the tropics, supporting a vast diversity of microscopic life. The microbe communities found in each tiny body ecosystem are known as ‘microbiota’, and their genomes as ‘microbiomes’.
Analysing these miniature ecosystems can be tricky, but techniques have advanced rapidly in recent years. One way to understand body bugs is to take swabs and samples and grow them in petri dishes. But this doesn’t show the whole story. Many microbes that live happily inside our bodies cannot survive on a dish in a lab, so we never see them. Now, with modern gene-sequencing techniques, it’s possible to detect the signatures of these hidden body bugs, and the results are astonishing. As it turns out, we’re more microbe than human, and our microbiomes are more unique than our genes.
Old estimates suggested that bacterial cells outnumbered our own cells ten to one. But recent science is a bit more conservative. It’s likely that there are between 30 and 50 trillion bacterial cells inside you right now, compared to just 30 trillion human cells. That means that we are only half human at best. What’s more, while
we humans share around 99.9 per cent of our DNA with each other, our internal ecosystems are nowhere near as similar. Compare our gut microbes and we only share between 80 and 90 per cent of the same genes.
Some scientists think that we should stop thinking of ourselves as individuals and start thinking of ourselves as ecosystems called ‘holobionts’ – a word that literally means ‘whole organism’. This approach helps to make sense of the massive impact our body bugs have on our health. Our microbes don’t just ride around inside us, they form an integral part of our biology; we need them as much as they need us.
Take the gut, for example. It’s the richest ecosystem in your body, thanks to you. The food you eat
provides a constant supply of nutrients, supporting well over three-quarters of the bacteria that call
you home. But they aren’t freeloading; gut microbes are a vital part of your digestive system. Bacteria have genes that we don’t, which allow them to do metabolic tricks that we can’t. This means that they can digest food that our bodies wouldn’t otherwise be able to break down. They can also make essential vitamins that we can’t produce on our own.
Having microbes in our intestines lets us extract much more nutrition from the food that we eat. In Japan, for example, some people have a gut bacteria species called Bacteroides plebeius. It comes from the seaweed in their diet, and it makes an enzyme that can digest complex sugar found in red algae. Without the enzyme, the sugar would just pass straight through. Thanks to the bacteria, the sugar becomes a new
source of energy.
Bacteria also seem to help keep the lining of our gut safe. Our intestines need to be able to absorb nutrients without allowing allergens, toxins and bad bacteria to get into the body. In mice, killing certain gut bacteria can trigger a peanut allergy. Restoring the bacteria reverses the effect. The bugs seem to be able to stop dangerous peanut proteins getting into the body.
The ecosystem in our intestines is essential for our survival. However, like any other ecosystem on the planet, maintaining a healthy community is a delicate balancing act. Our guts support hundreds of species, and if this complex mix gets out of balance, it can spell disaster. Problems with themicrobiome have been
linked to all kinds of diseases, from acne and diarrhoea to diabetes and cancer.
Work to unpick the role microbes play in our health is still in its early stages, but there are some links that are starting to become clearer. Genetics and lifestyle both have important roles to play in how we interact with our gut microbes.
One way that scientists have been learning about this is by looking at mouse poo. Analysing the leftovers of a meal can tell us how much energy the mice have managed to extract from their diet. When mice have gene-mutations linked to obesity, everything changes. Mice with one type of mutation were able to get more calories from the same amount of food. Mice with a different mutation wanted to eat more. In both cases, it seemed that their gut bacteria were partly to blame.
Gut bacteria are also in tune with our lifestyles. Eating lots of meat makes them switch on protein-digesting enzymes,while crunching through vegetables encourages them to turn on enzymes that break down complex carbohydrates. This is hugely beneficial for both us and the microbes.
We can’t digest all the fibre from plant-based foods on our own, and without fibre, good bacteria can’t survive. So we work together. The more fibre you eat, the more bacteria you can support, and there are benefits for you too. Fibre-eating bacteria make fatty acids, which nourish your gut cells, help to maintain your gut barrier and reduce inflammation.
So far, the focus has mostly been on the intestines. They tend to get all the attention when it comes to microbes because they’re easily the richest ecosystem in your body. This is simply because the gut is full of nutrients. However, just because other parts of the body don’t have as much food for the microbes to eat
doesn’t mean they don’t have their own microscopic communities.
Take the skin, for example. Adults have around two square metres of skin, a huge habitat for microbes. Most of it is cool and slightly acidic, a tricky place for microbes to make their home, but there are havens among these deserts, especially in folds and crevices.
Warm, damp creases in the skin, like the belly button, attract bacteria like Staphylococcus aureus. Glands, like those on the face, attract species like Propionibacterium acnes, named after the spots it causes when it grows too fast. Each little crack forms a tiny ecosystem with its own unique community of body bugs.
Different sets of microbes live inside the ear, on the edge of the nose, up the nostril, in the armpit and in the crease of the elbow. They help to protect the skin from colonisation by more dangerous, disease-causing bacteria and fungi. They also coach the immune system, training it to prevent infections by parasites.
These microbes also form part of your unique body-bug fingerprint. The bacteria that colonise our hands leave telltale traces on everything we touch. The signatures they leave behind are so specific that not only can you tell which person touched an object, you can even tell which finger. Your miniature ecosystems are as unique as you, and without them, you wouldn’t be here.