BioMed
Ed Yong draws on laboratory reporting to explore animal-microbe partnerships, including skin bacteria that attract mosquitoes, and how their effects depend on context.
Broad attacks on microbes kill the many helpful species along with the few harmful ones, like nuking a city to deal with a rat. A long campaign against microbes has built a world hostile to the ones we depend on.
We constantly shed a personal cloud of microbes, leaving a mark on everything we touch and every space we pass through. Within a day of moving into a new home, our microbes overwrite it so the place reflects its occupants. Our microbial selves reach beyond our skin into the world around us.
What looks like a single creature is really a community of many species living together in one body. Every animal, from a person to a squid to a wasp, carries millions of microbes, and in humans the microbial cells may outnumber the human ones. Even the cells we call human are part microbe, which blurs where the body ends and its residents begin.
Whether a microbe helps or harms depends on the context and on the host it lives in. The same community that is mostly beneficial can turn harmful under different conditions. So sorting microbes into friends and enemies misses how their roles shift.
Only a tiny fraction of microbes can make us sick: around 100 species of bacteria cause infectious disease in humans. Hundreds of thousands of other species live peacefully inside us, mostly in the gut. Treating microbes chiefly as enemies to wipe out gets the numbers backwards.
The microbial community on one part of a body differs more from another part of the same body than from the same part on another person. A body is less one microbial habitat than a set of distinct neighbourhoods, each with its own residents.
Symbiosis, different organisms living together, is the normal condition of animals, not an exception. Seeing bodies as webs of dependent creatures challenges the picture of living things as isolated individuals whose natural state is competition. The author also warns against turning symbiosis into a sentimental story of pure harmony.
Resident microbes help shape organs, break down food, train the immune system, and guard against disease. They are not one extra organ but many partners working through many routes, covering digestion, defence, growth and development.
In mouse experiments, changing an animal's microbes changed how it acted, making it more or less anxious and more or less depressed. This suggests behavior is partly shaped by an animal's microbial residents, not only by its own body and brain.
A host's microbes can change how other creatures respond to it. Some skin bacteria make a person more attractive to malaria-carrying mosquitoes, while others drive the mosquitoes away. They can also change how the body responds to medicines, such as some cancer drugs.