THAT'S THE WHY:You might like to think you are you. And, in many ways, of course you are. But when it comes to the number of cells that call your body home, only about 10 per cent of them are actually yours.
The rest are mostly yeast and bacteria, a crew of hangers-on that like to dwell on the various habitable surfaces you offer – perhaps on your skin, inside your mouth or nose or within the depths of your bowels.
The bacteria that make up your gut microbiota have been attracting particular interest in recent years – and thanks to the advent of molecular methods to profile them, the sheer diversity, scale and function of what’s in there is becoming more apparent.
Some of these bugs are involved in nutrient absorption from your food, and research is uncovering links between your gut bugs and your immune system.
Humans are not alone in fostering healthy relationships with bacteria. A paper published earlier this year in BioEssays points out that such arrangements are widespread in the animal kingdom, even among relatively simple organisms such as the freshwater Hydra.
Experiments found that different Hydra species hosted different complements of bacteria, and where the bacteria were artificially removed, the animals failed to grow.
The authors conclude that “beneficial bacterial-host interactions should be considered an integral part of development and evolution”.
