The human gut contains a large number of different types of bacteria, called the microbiome. The composition of the microbiome is known to be affected by many factors, such as what you eat, and in recent years has increasingly been associated with a range of health issues, like obesity.

 Antibiotics are known to cause large changes in the microbiome by killing off many of the bacteria, and can lead to illness when pathogenic bacteria take over (like Clostridium difficile infections).

The authors of this Nature study have now systematically looked at the effects of many other types of medications (non-antibiotics) on a panel of bacteria representing the gut microbiome.

They found that over a quarter of the drugs tested could stop gut bacteria form growing. The effects might even be larger, as the researchers had to make assumptions about the drug concentrations in the gut.

They also found that people taking some of the drugs had side effects consistent with side effects seen with taking antibiotics. Of greatest concern is that they also found that these non-antibiotic drugs might help cause antibiotic resistance, which is an increasing threat to our ability to treat infections.

Gut bacteria play a fundamental role in the activation of some fighter cells, which help our immune system to function properly. The use of oral antibiotics is known to disturb the balance of bacteria in the gut by killing off some bacteria, and the work by Maier and colleagues has shown that this effect is not specific to antibacterial drugs.

By testing the effect of drugs on 38 different bacterial species that are present in the gut, many of which are not used in screening of drugs due to their non pathogenic nature, it was possible to identify non-antibacterial drugs that were able to kill some species. This approach provides an opportunity to develop new antibiotics from non-antibacterial drugs to help tackle the global issue of antibiotic resistance.

The study also found that bacteria that were resistant to antibacterial drugs were more likely to be resistant to non-antibacterial drugs suggesting that these antibacterial drugs and non-antibacterial drugs affect bacteria in similar ways. This correlation is not the same as saying the use of these non-antibacterial drugs leads to or causes antibiotic drug resistance but highlights an area of interest that should be watched until more data becomes available.

Finally it is important to stress that the studies were carried out in isolation in plates that contained at any one time no more than one bacterial species and one drug. Biological systems such as our guts are much more complicated with tens of trillions of bacterial cells in competition and where there are complex relationships between bacteria, acid levels in the gut, biomolecules such as carbohydrates, enzymes, hormones and others.

An important question that is yet to be answered is therefore whether these findings can be reproduced in holistic biological systems. 

Our ability to accurately screen and analyse the trillions of bacteria that inhabit our gut is increasing at a rate that is almost unfathomable. With this increasing capacity comes a tsunami of microbiome-related research output, and an increasingly long list of diseases with which the microbiome is linked. However, our ability to truly understand the intricacies of how the microbiome co-exists with its host is inherently limited by our relatively poor ability to culture them in a laboratory setting.

New research published in Nature Microbiology is now shedding light on the gastronomic preferences of our gut bacteria, enabling much more effective methods of laboratory culture, thus helping researchers understand the unique idiosyncrasies of our gut bacteria. The group from Heidleberg were surprised by a number of their findings, showing that many additives traditionally thought to promote growth actually inhibited bacterial growth. This is an important step forward in our scientific endeavor to understand the microbiome and its impact in disease development and therapeutic intervention.

In line with these findings, new research also fro Heildelburg has shown that bacteria are not only sensitive to culture additives, but a number of commonly prescribed drugs. It has always been known that anti-biotics negatively affect the good bacteria that reside in our gut, but this study has shown that a number of non-antibiotic drugs also affect the viability of certain bacteria. Although only shown in vitro, without the complexities of the human body, results showed that a number of antidiabetic, anti-inflammatory and antipsychotic medication affected bacterial viability.

These results are critically important as it suggests that a wider group of medications may also be driving antibiotic resistance, a looming threat to the health of our current society.