Human cells make up only 43% of the body’s total cell count. The rest are microscopic colonists.
Understanding this hidden half of ourselves – our microbiome – is rapidly transforming understanding of diseases from allergy to Parkinson’s.
The field is even asking questions of what it means to be “human” and is leading to new innovative treatments as a result.
“They are essential to your health,” says Prof Ruth Ley, the director of the department of microbiome science at the Max Planck Institute, “your body isn’t just you”.
No matter how well you wash, nearly every nook and cranny of your body is covered in microscopic creatures.
This includes bacteria, viruses, fungi and archaea (organisms originally misclassified as bacteria). The greatest concentration of this microscopic life is in the dark murky depths of our oxygen-deprived bowels.
Prof Rob Knight, from University of California San Diego, told the BBC: “You’re more microbe than you are human.”
Originally it was thought our cells were outnumbered 10 to one.
“That’s been refined much closer to one-to-one, so the current estimate is you’re about 43% human if you’re counting up all the cells,” he says.
But genetically we’re even more outgunned.
The human genome – the full set of genetic instructions for a human being – is made up of 20,000 instructions called genes.
But add all the genes in our microbiome together and the figure comes out between two and 20 million microbial genes.
Prof Sarkis Mazmanian, a microbiologist from Caltech, argues: “We don’t have just one genome, the genes of our microbiome present essentially a second genome which augment the activity of our own.
“What makes us human is, in my opinion, the combination of our own DNA, plus the DNA of our gut microbes.”
t would be naive to think we carry around so much microbial material without it interacting or having any effect on our bodies at all.
Science is rapidly uncovering the role the microbiome plays in digestion, regulating the immune system, protecting against disease and manufacturing vital vitamins.
Prof Knight said: “We’re finding ways that these tiny creatures totally transform our health in ways we never imagined until recently.”
It is a new way of thinking about the microbial world. To date, our relationship with microbes has largely been one of warfare.
Antibiotics and vaccines have been the weapons unleashed against the likes of smallpox, Mycobacterium tuberculosis or MRSA.
That’s been a good thing and has saved large numbers of lives.
But some researchers are concerned that our assault on the bad guys has done untold damage to our “good bacteria”.
Prof Ley told me: “We have over the past 50 years done a terrific job of eliminating infectious disease.
“But we have seen an enormous and terrifying increase in autoimmune disease and in allergy.
“Where work on the microbiome comes in is seeing how changes in the microbiome, that happened as a result of the success we’ve had fighting pathogens, have now contributed to a whole new set of diseases that we have to deal with.”
The microbiome is also being linked to diseases including inflammatory bowel disease, Parkinson’s, whether cancer drugs work and even depression and autism.
Obesity is another example. Family history and lifestyle choices clearly play a role, but what about your gut microbes?
This is where it might get confusing.