Who's already invited to dinner?

Mysterious hitch-hikers

Every healthy person carries a kilo and a half of microbial mass in their intestines, hundreds of different types of microbe busily making a living in this nutritious environment. That is a population of roughly a trillion munching away in the dark (assuming a microbe weighs about a picogram). It is relatively easy to see them with a combination of sampling bravado and microscopy, but to do more than peer curiously is in most cases impossible: we can only to study the few that grow in laboratory culture. There is nothing mysterious about the fact that most intestinal bacteria have not yet been cultivated. Shaker flasks and culture media are only artificial systems that do not mimic the conditions these creatures need.

Whether cultivatable or not, the gut flora and fauna are either autochthons or transients just passing through; our passenger list varies with our age, our diet, our health and our lifestyle. Our symbionts swarm and grow over the interface that separates what we are from what we eat. Mostly, they help us, and to a surprising degree.

Many hands make your lights work

Xu and Gordon look at our residents as a hardworking and capable team specializing in a dirty but necessary job. These aren't parasites: they form a machine within us like a liver or a heart. The microbes coexist and/or cooperate to carry out complex chemistry just like liver cells and, just like liver cells, they benefit directly when they contribute to our health in this way. Our gastrointestinal tracts work better with them than without; the microbes assist in the breakdown of starches and proteins and also produce vitamins and easily absorbed specialized fatty acids.

Tiny doesn't mean ineffectual

The growth, behaviour and death of gastrointestinal microbes are the result of interactions with their cohabitants but also with us: our intestines are their entire world; we are their habitat. We must not, in our multicellular arrogance, forget that they form part of our environment too: they can influence our growth, behaviour and death in ways more subtle than food poisoning. Gordon and Xu mention the shift in human diet during infancy from milk to solid foods, a point where an invasion of colonial microbes set up camp within us with drastic consequences. They stimulate the correct development of intestinal microstructures, teach our nascent immune systems to recognize microbial friend from microbial foe, and assist our innards in breaking down our tough new diet. Without the help and training, we cope poorly with the world. Without us, their fates are uncertain. Together as an ecology, we're quite a team.