(7) Investigating resistant bacteria in the gut

Some bacteria in our gut are resistant to almost all antibiotics. If they cause an infection, there are very few treatment options left. As part of NRP 79, a research group led by Andrea Endimiani of the University of Bern looked for ways to remove these so-called superbugs from the gut before they can cause harm. As an alternative to costly and ethically problematic mouse experiments, the research team developed a new in vivo model using insect larvae.

Testing strategies to remove superbugs from the gut requires in vivo models: living organisms in which researchers can observe how bacteria behave and how they react to potential treatments. Mouse models are currently the gold standard for this kind of research. However, they have significant drawbacks: results from mice cannot always be transferred to humans, the experiments are expensive and labour-intensive, and there are important ethical concerns about using rodents in research.

Insects offer a promising alternative – they are easier to keep, less costly and raise far fewer ethical issues. The research team therefore colonised the gut of Zophobas morio larvae – a type of large mealworm – with multidrug-resistant bacteria commonly found in hospital patients, and observed how the bacteria behaved and whether they could be eliminated using a special mixture of bacteriophages (viruses that attack bacteria). The results were then compared with those from the standard mouse model.

The project had three goals:

• To find out whether Zophobas morio larvae can be reliably colonised with three clinically important superbug species (Escherichia coli, Klebsiella pneumoniae and Salmonella enterica).

• To study how the larvae’s own gut bacteria change when infected by superbugs and to test whether a bacteriophage cocktail can clear the superbugs out again.

• To compare the newly established larvae model directly with the standard mouse model.

The larvae could indeed be colonised with all three superbug species. When treated with the bacteriophage cocktail, it successfully cleared the bugs that were sensitive to the phages, but the cocktail had no effect on phage-resistant strains – a realistic reflection of what happens in clinical practice. The team developed a shorter, 14-day protocol to test the effect of decolonization strategies (for instance bacteriophages) in Z. morio larvae.

In one of the most striking findings, the team also tested bacteriophages as a preventive treatment: When larvae and mice received phages before being exposed to the superbug, the bacteria could not establish themselves in the gut. The larvae model was successfully validated against the mouse model – the two models showed broadly comparable outcomes, supporting the larvae model as a credible alternative.

The new larvae model offers a number of advantages over mouse experiments: It is cheaper, faster, easier to handle, allows much larger numbers of test subjects and raises far fewer ethical concerns. It does not replace the mouse model entirely, but could serve as a first screening step – allowing researchers to test many possible strategies in larvae and only move the most promising ones on to mice. This would significantly reduce the number of animals used in research while speeding up the search for new ways to fight antibiotic resistance.

Using Zophobas morio larvae to design a new in vivo model of intestinal colonization due to multidrug-resistant Enterobacterales