The Antibiotics that Could Kill You

In 2010, Americans were prescribed 258 million courses of antibiotics, a rate of 833 per thousand people. Such massive usage, billions of doses, has been going on year after year.
We have few clues about the consequences of our cumulative exposures. We do know that widespread antibiotic treatments make us more susceptible to invaders by selecting for resistant bacteria.
These risks are now well-known, but I want to lay out a new concern: that antibiotic use over the years has been depleting the pool of our friendly bacteria — in each of us — and this is lowering our resistance to infections. In today’s hyperconnected globe, that means that we are at high risk of future plagues that could spread without natural boundaries from person to person and that we could not stop. I call this “antibiotic winter.”
To explain: In the early 1950?s, scientists conducted experiments to determine whether our resident microbes — the huge number of bacteria that live in and on our bodies, now called our “microbiome” — help in fending off invading bacteria. They fed mice a species of a typical invader, disease-causing salmonella. It took about 100,000 organisms to infect half of the normal mice. But when they first gave mice an antibiotic, which kills both good and bad bacteria, and then several days later gave them salmonella, it took only three organisms to infect them. This isn’t a 10 or 20% difference; it’s a 30,000-fold difference.
That was in mice, but what about humans? In 1985, Chicago faced a massive outbreak of salmonella. At least 160,000 people became ill and several died from drinking contaminated milk. The health department asked victims of the outbreak and unaffected persons, “Have you received antibiotics in the month prior to becoming ill?” People who said yes were five times more likely to become ill than those who drank the milk but hadn’t recently received antibiotics.
People carry a small number of highly abundant bacterial species and a large number of much less common ones. For example, you may carry trillions of Bacteroides thetaiotaomicron in your colon and only a thousand cells, or fewer, belonging to many other species. We are not sure how many rare species any of us has. If you had only 50 cells of a particular type, it would be difficult to detect them against the background of trillions of others.
When you take a broad-spectrum antibiotic, which is the kind most commonly prescribed, it may be that rare microbes occasionally get wiped out entirely. And once the population hits zero, there is no bouncing back. For your body, that species is now extinct. My worry is that some of these critical residential organisms — what I consider “contingency” species — may disappear altogether.
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