What made antibiotics so wildly successful was the way they attacked bacteria while sparing us. Penicillin, for example, stops many types of bacteria from building their cell walls. Our own cells are built in a fundamentally different way, and so the drug has no effect. While antibiotics can discriminate between us and them, however, they can’t discriminate between them and them–between the bacteria that are making us sick and then ones we carry when we’re healthy. When we take a pill of vancomycin, it’s like swallowing a grenade. It may kill our enemy, but it kills a lot of bystanders, too. (...)
Each of us is home to several thousand species. (I’m only talking about bacteria, by the way–viruses, fungi, and protozoans stack an even higher level of diversity on top of the bacterial biodiversity.) My own belly button, I’ve been reliably informed, contains at least 53 species. Many of the species I harbor are different than the ones you harbor. But if you look at the kinds of genes carried by those species, our microbiomes look very similar. That’s partly because surviving on a human body requires certain skills, so any species that is going to last long in your lungs, say, will need many of the same genes.
But the similarity speaks to something else. The microbiome keeps us healthy. It breaks down some of our food into digestible molecules, it detoxifies poisons, it serves as a shield on our skin and internal linings to keep out pathogens, and it nurtures our immune systems, instructing them in the proper balance between vigilance and tolerance. It’s a dependence we’ve been evolving for 700 million years, ever since our early animal ancestors evolved bodies that bacteria could colonize. (Even jellyfish and sponges have microbiomes.) If you think of the human genome as all the genes it takes to run a human body, the 20,000 protein-coding genes found in our own DNA are not enough. We are a superorganism that deploys as many as 20 million genes.
It’s not easy to track what happens to this complex organ of ours when we take antibiotics. Monitoring the microbiome of a single person demands a lot of medical, microbiological, and genomic expertise. And it’s hard to generalize, since each case has its own quirks. What happens to the microbiome depends on the particular kind of bacteria infecting people, the kind of antibiotics people take, the state of their microbiome beforehand, their own health, and even their own genes (well, the human genes, at least). And then there’s the question of how long these effects last. If there’s a change to the microbiome for a few weeks, does that change vanish within a few months? Or are there effects only emerge years later?
Scientists are only now beginning to get answers to those questions. In a paper just published online in the journal Gut, Andres Moya of the University of Valencia and his colleagues took an unprecedented look at a microbiome weathering a storm of antibiotics. The microbiome belonged to a 68-year-old man who had developed an infection in his pacemaker. A two-week course of antbiotics cleared it up nicely. Over the course of his treatment, Moya and his colleagues collected stool samples from the man every few days, and then six weeks afterwards. They identified the species in the stool, as well as the genes that the bacteria switched on and off.
What’s most striking about Moya’s study is how the entire microbiome responded to the antibiotics as if it was under a biochemical mortar attack. The bacteria started producing defenses to keep the deadly molecules from getting inside them. To get rid of the drugs that did get inside them, they produced pumps to blast them back out. Meanwhile, the entire microbiome powered down its metabolism. This is probably a good strategy for enduring antibiotics, which typically attack the molecules that bacteria use to grow. As the bacteria shut down, they had a direct effect on their host: they stopped making vitamins and carrying out other metabolic tasks.
by Carl Zimmer, National Geographic | Read more:
Photo: Health Research Board on Flickr via Creative Commons
