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What if gut-bacteria communities "remember" past antibiotic exposures?

The number of beneficial microbes inhabiting each of our intestines is on the order of the number of synaptic connections in the human brain. That's a huge number.

And it's clear that, in with any complex ecosystem, these myriad individual life-forms are all "talking" to one another, albeit by chemical signals and physical interactions rather than speech. In doing so, they expand the variety of ways in which they help us humans digest our food, help us maintain our weight, deter invasions by nasty pathogens, and even fine-tune the immune response, probably fending off autoimmune disease and allergies when all goes well.

If these friendly fellow travelers are signaling back and forth like neurons in a brain, is it all that farfetched to imagine that, as a community, they retain a "memory" of their experience? Apparently not, as this just-published study from Stanford gut-bug chroniclers David Relman, MD, and Les Dethlefsen, PhD, shows.

In an earlier study, the researchers gave subjects a single five-day course of Cipro - an antibiotic considered pretty benign to beneficial bacteria because patients to whom it's given don't typically experience gastrointestinal side effects. After initially being thrown for a loop, the subjects' enteric microbes bounced back within about a week to an equilibrium pretty close to what it had been before the round of Cipro.

In this study, which appears online in the Proceedings of the National Academy of Sciences, Relman and Dethlefsen served up not one but two courses of Cipro, separated by a couple of months. Again, their subjects' internal ecosystems were largely restored to their initial healthy state within a week or so after the first antibiotic administration. The second time around, however, those communities never did return to their previous equilibrium. There were no outwardly visible side effects, which is good, but as Relman told me, "Visible side effects are the last thing to show up."

The researchers are concerned about potential nasty surprises down the road, due to the observed replacement of numerous species of microbe, after Cipro administration, by other, closely related species. (From an ecological perspective, it might look like the same tree, but when you look closely many of the twigs and leaves are in different places.) With every vanished bug species comes, potentially, a vanished valuable function - such as the secretion of a chemical that chases off an invading pathogen. The replacement bug may by similar to the replaced bug in every respect but this one lost function. So the loss wouldn't be noticed... until, perhaps decades later, the pathogen shows up in the GI tract.

Not that a person who's sick shouldn't take antibiotics. "I don't think we're ever going to quit using them," Dethlefsen told me. "Their benefits are too great. If you need them, take them!" Instead, he's hoping we'll eventually learn enough about our myriad one-celled residents to know which ones will need replacing, supplementing, or encouragement after an antibiotic course, so we can clean up after the antibiotic elephant.

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