Two studies analyze the aftermath of antibiotics

As scientists learn more about the critical roles played by the friendly microbes living in our bodies, the field has started to wonder how antibiotic use, especially recurrent use, affects these bacteria and our health. Now, two studies, one in humans and one in mice, offer more insights about these drugs’ aftereffects. The findings suggest antibiotics can cause some disruption to the composition of the gut microbiome, as well as alter the activity of some immune cells.

In a study performed in 12 healthy adults, researchers from Denmark, Germany, and China found that the composition of the participants’ gut microbiomes almost completely recovered from a course of last-resort antibiotics, such as vancomycin, within 1.5 months of treatment (Nat. Microbiol. 2018, DOI:10.1038/s41564-018-0257-9). But a few bacterial species didn’t come back by the end of the study—about 180 days after treatment. Author Filip K. Knop of University of Copenhagen says this composition change could impact gastrointestinal health, but more studies are needed. Moreover, the current study only looked at the antibiotics’ effects after a single course of treatment, he says, and researchers need detailed data after multiple courses.

In the mouse study, U.S. and U.K. researchers also report that antibiotics can disrupt gut microbe composition, but this team finds that the disruption may affect the behavior of some immune cells (Sci. Transl. Med. 2018, DOI:10.1126/scitranslmed.aao4755). This immune cell dysfunction could lead to inflammatory conditions, the researchers say.

Immune cells called macrophages help fight off bacteria and other pathogens in our body. In the gut, the immune response of these cells is usually dampened so they can live side by side with our friendly gut bacteria. Elizabeth Mann of University of Manchester and coworkers found that mice exposed to antibiotics for seven days develop hyper-responsive macrophages, which remain dysfunctional 60 days after a treatment, a period equivalent to years in humans.

Further experiments revealed that the affected animals also had lower levels of short-chain fatty acids (SCFAs) in their guts, compared with unaffected animals. When the researchers supplemented the mice’s diets with SCFAs during antibiotic treatment, they prevented the macrophage dysfunction.

Mann says a striking similarity between her team’s mouse study and the human study is that Bifidobacteria species go missing after antibiotic treatment. Bifidobacteria species help produce SCFA in the gut, and those fatty acids are known to dampen the activity of gut immune cells.

Willem M. De Vos, a microbiologist at Wageningen University & Research who was not involved in either study, says the results of the human study are not surprising, but that researchers need to investigate the effects of recurrent use of antibiotics, especially of more common classes, such as macrolides. He adds that the mouse study highlights the important interactions between gut bacteria and the immune system.