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.
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.”
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”.
Goldmine of information
I met Dr Trevor Lawley at the Wellcome Trust Sanger Institute, where he is trying to grow the whole microbiome from healthy patients and those who are ill.
“In a diseased state there could be bugs missing, for example, the concept is to reintroduce those.”
Dr Lawley says there’s growing evidence that repairing someone’s microbiome “can actually lead to remission” in diseases such as ulcerative colitis, a type of inflammatory bowel disease.
Prof Knight said: “It’s incredible to think each teaspoon of your stool contains more data in the DNA of those microbes than it would take literally a tonne of DVDs to store.
“At the moment every time you’re taking one of those data dumps as it were, you’re just flushing that information away.
“Part of our vision is, in the not too distant future, where as soon as you flush it’ll do some kind of instant read-out and tells you are you going in a good direction or a bad direction.