For more than a century we have been engaged in a war on bacteria. We deploy an arsenal of antibiotics, hand sanitisers, pasteurisation and food regulations to tackle the moulds and bacteria and so, we hope, hold off disease and death.
I grew up on that field of battle. My mother instilled in our family a deep fear of botulism, and countless other unnamed germs possibly lurking in our food.
A touch of white on a wedge of cheese was enough to condemn it.
The slightest dent in a can of food consigned it to the rubbish, no matter that the dent came from being dropped on the floor. You never know, could be botulism; better safe than sorry.
In the decades since Louis Pasteur discovered bacteria, medical research has focused mainly on their role in causing disease.
The bacteria that reside in and on our bodies were generally regarded as either harmless freeloaders, or pathogens to be defended against.
But then in the early 2000s, researchers discovered hundreds of new species of bacteria in the human gut doing all sorts of unexpected things.
To their surprise, microbiologists discovered that we are made up of 90 per cent bacteria. Nine out of every ten cells in our bodies are not human but belong to these microbial species (most of them residents of our gut).
As one scientist put it to me, we ‘stand on the verge of a paradigm shift in our understanding of health as well as our relationship to other species’.
Metaphors about it being a ‘war’ no longer made much sense.
So what exactly are the 500 or so distinct species and countless different strains of those species that make up the kilogram or so of microbes in our gut doing there?
For most of these microbes, their survival depends on our own, and so they do all sorts of things to keep their host – us – alive and well.
Perhaps their most important function is to maintain the health of the gut wall, or epithelium. In the course of a lifetime, 60 tons of food pass through the gastrointestinal tract, an exposure to the world that is fraught with risk.
It appears that much of that risk is managed, most of the time brilliantly, by the gut bugs.
Taken as a whole, the organisms in the gut constitute the largest and one of the human body’s most important organs of defence.
So why would the body enlist bacteria in all these critical functions, rather than evolve its own systems to do this work? One theory is that because microbes can evolve rapidly they can respond with much greater speed and agility to changes in the environment.
Though we’ve tended to think of bacteria as agents of destruction, they are invaluable creators as well. Gut bugs manufacture essential vitamins (including vitamin K as well as several B vitamins) and a great many other compounds scientists are only just beginning to recognise.
Some of these compounds act on the central nervous system, moderating our appetite and the mechanisms that determine how we store fat.
So might changing the composition of our gut bacteria in turn change our weight?