When we take medications, we generally do two things:  first, we swallow some pills, then we wait for them to kick in. Whether or not they do, however, may be tied to our gut microbes.

Intestinal bacteria influence the availability and activity of therapeutic drugs in the body. For instance, some bacteria metabolically convert, or ‘biotransform,’ drugs into their active forms; others inactivate them. And some, according to a new report published in Nature, don’t chemically manipulate drug molecules — they hoard them.

In this study, researchers incubated 25 representative strains of gut bacteria with 12 orally administered drugs, including those used to treat asthma, high cholesterol, and diarrhea. By measuring drug levels in the growth medium before and after 48 hours of incubation, the scientists identified 29 novel bacteria-drug pairs in which the drug was depleted from the medium. Comparing drug concentrations in the medium alone with that of the total culture revealed that, in most cases, the drug was absent from the medium but recoverable from the total culture. These results suggest the medications were accumulating inside the bacteria.

The question is: When bacteria vacuum up drug molecules, does this alter the drug’s effect on the host?

To explore this, the researchers incubated Caenorhabditis elegans, a nematode and model organism, with duloxetine, an antidepressant that was accumulated by several bacterial strains. While duloxetine alone decreased nematode motility, adding a duloxetine-accumulating strain of E. coli to the culture reduced this effect.

These findings indicate that bacterial hoarding of medications may affect the way those drugs affect their targets. Ultimately, more research is required to determine whether a similar scenario plays out in the human gut, and in the context of other drugs. Greater insight into the interplay between medications and gut microbes could expand our understanding of drug bioavailability and efficacy, and how they may vary from one person (and gut microbiome) to the next.