It's still quite a mystery to scientists why certain drugs work well in some people and are completely ineffective in others. Work by a team of researchers from Yale University suggests a person's unique gut bacteria population could affect how different oral drugs are metabolized, and a new study, for the first time, has measured how a number of different bacteria interact with over 200 common medications.

Only recently have scientists begun to investigate the effects of the gut microbiome on the efficacy of different therapeutic drugs. The most advanced area of study has been connecting gut bacteria to the success, or failure, of new types of cancer treatments.

We know that certain drugs can interact with specific microbes in the gut to result in either heightened activity, inactivity or even toxicity. The new research set out to systematically catalog these specific drug-microbe interactions looking at 76 kinds of gut bacteria and how they individually interact with 271 common oral therapeutic drugs.

Strikingly, the results showed 176 of the examined drugs were negatively affected by at least one bacterial strain. It was discovered that tracking the direct presence of a specific bacterial species was not the most effective way to predict how much a drug's activity would be effected. Instead, it was the presence of certain enzymes produced by the microbes that could better predict how a drug would be influenced.

"The work is a first step in identifying biomarkers that could help doctors prescribe the drugs that are the safest and most effective for individual patients," explains Maria Zimmermann-Kogadeeva, co-lead author on the new study.

As well as offering a potential library of biomarkers for clinicians to detect, allowing more personalized drug prescriptions for patients, the research points to ways the microbiome could be manipulated to make certain treatments more effective. The long-term goal being to alter a patient's microbiome so it beneficially amplifies the effect of certain drug treatments.

"We hope this study provides a useful first step in understanding the microbiome contribution to drug metabolism," says Michael Zimmermann, co-lead author of the study. "We think these approaches could shed light on how the gut microbiome also modulates our response to non-drug compounds, such as dietary nutrients and environmental agents."

The new study was published in the journal Nature.