The microbes living on and in you can change your mood, your mind and your health – challenging our ideas about human nature.
You may, quite reasonably, think you are an individual of the species Homo sapiens. Once you have finished reading what follows, you will hopefully have been convinced that there is far more to us than that. Trillions of other organ-isms live on (and, more notably, in) your body. As you will see in the reports that follow, their impact on you is such that you will probably never think about yourself in the same way again. Your microbes change who you are and what it means to be you. With knowledge of this facet of ourselves growing rapidly, exploring it has never been more relevant.
Until recently, scientists believed that there were three discrete parts of our nature that reflected solid aspects of an individual self: the immune system, the genome and the brain. “None of these pillars of the traditional definitions of the self – immunity, genome integrity, the central nervous system – are free of microbial impact,” says Thomas Bosch at Kiel University in Germany.
The microbes that colonise us, collectively known as the microbiome, challenge the concept of a discrete self. These include bacteria, viruses and fungi, although the bacteria are the best-studied. Together, these microbes orchestrate the immune system, influence how the brain works and grows, and affect our personality and feelings. More than a third of the genes in the human genome have a bacterial origin, and the action of trillions of microbial genes in the gut has direct impacts on our nutrition and metabolism.
Recognising this leads to the startling conclusion that a discrete human body isn’t an individual, it is a ‘metaorganism’. Bosch, who runs a metaorganism research centre, is one of a growing number of scientists who say the key elements that we believe make us individuals are themselves produced by interactions with microorganisms. If this is the case, we need to rethink what it means to be human.
“Certainly, the line that demarcates where we end and the external world begins is up for debate,” says Geraint Rogers at the South Australian Health and Medical Research Institute in Adelaide.
This blurring of boundaries makes sense, given that we evolved on a microbial planet and that microbes were here long before us. “Fundamentally, we have always been part microbial,” says Alan Walker at the University of Aberdeen, UK. This view has important implications for our position in nature, but also means we will need to change the way we treat ourselves when we are ill.
“We need to adapt to a holistic way of thinking that accounts for the full complexity of the interconnection between our gut, our mind and the trillions of microbes that live in our gastrointestinal system,” says Harriët Schellekens at University College Corkin Ireland.
“Fundamentally, we have always
been part microbial.”
At the moment, however, we don’t always know how to intervene and make adjustments: the system is just too complicated, with many unknowns. The best microbial-based treatment we have comes from using faecal transplants to treat recurrent infections of the bacterium Clostridioides difficile.
Other than that, reliable, clinical evidence for a microbiome-based approach in medicine is only slowly accumulating. In one promising example, a trial in the UK looked at premature babies at risk of developing a dangerous, often fatal bowel condition called necrotising enterocolitis. The rate of the condition fell by more than half in babies given a daily dose of live, protective bacteria.
Still, while the promise is huge, treatments that exploit the microbial com-munity inside us have yet to catch up. Take the link between the microbiome and mental health. Our gut microbes produce many of the same neurotransmitters that are made in the brain, and use them to communicate with it. We all experience the intimate communication between the brain and the gut when we feel nervous, or butterflies in our stomach, but the connection goes deeper. Several studies have shown a correlation between a lack of certain gut bacteria and depression, as well as with anxiety –but we don’t know if that link is causal. In rodent trials, faecal transplants from people with depression to rats seem to induce depressive behaviour in the rats. And in preliminary trials, when people with depression were treated with faecal transplants, their symptoms have improved.
Most transplant successes have been in conjunction with regular medical treatment and the improvement in symptoms isn’t permanent. The long-term consequences of faecal transplants are also unclear. It all means that reliable treatments are still a way off. “For me, this area is a bit ‘watch this space’ –I wouldn’t be treating depression by treating the microbiome,” says Walker, who adds that this is certainly not some-thing people should self-administer.
For his part, Rogers says that the microbiome clearly has the capacity to explain differences in clinical outcomes between individuals. We take into ac-count genetic predispositions and existing conditions when we treat someone, he says, and it is increasingly clear that we should add the microbiome to that list.
All this has profound implications for what it means to be human. For hundreds of years, people have considered themselves apart from nature, which has led to an unfolding crisis of overexploitation of the environment. “Humans were always thinking they were different, they were better than nature, they looked at nature as something foreign,” says Bosch. “We have to go back to thinking that we are all part of an integrated, living cosmos. We are not better or separate – if we look in our own body, we are full of nature.”
