A fruit fly is only two millimeters long and lives for two months. Despite its short lifespan, it has immune cells that can eat harmful germs – similar to human macrophages. The fruit fly is also equipped with antimicrobial proteins and harbors a small microbiome of several thousand bacteria in the intestine.

Why does nature have such complex defense strategies for living things that are very short-lived? “An insect has to survive its larval stages, become sexually mature, and reproduce,” explains biology professor Jens Rolff from Freie Universität Berlin. “However, many bacteria have generation times of only 20 minutes. Their life cycle is much shorter. Having an immune system is definitely worthwhile for fruit flies!”

How does the immune system of an insect react to infection with harmful germs, so-called pathogens? How do intestinal flora and host interact? And how does this interaction affect the evolution and possible development of resistance in pathogens? Currently, little is known about these interactions. Seven working groups, headed by Jens Rolff, are now addressing these questions using as examples the fruit fly, honeybee, beewolf, wax moth, flour beetle, and cockroach.

The German Research Foundation (DFG) is funding the project “Integrating Insect Immunity, Microbiota and Pathogens” for up to eight years with roughly 3.7 million euros. In addition to scientists from Freie Universität, colleagues from the Swiss Federal Institute of Zurich (ETH), the University of Mainz, and the Regional Institute for Apiculture and Bee Biology in Hohen Neuendorf (e.V.) in Germany are also involved in the research project.

Jens Rolff’s team focuses on the parasitic wax moth Galleria. It attacks beehives and lays its eggs in the honeycomb. “Like most insects, wax moths pupate and dissolve almost all of their organs in order to rebuild them. Its gut, which is full of microbes, is completely renewed.”

If a person has an intestinal breakthrough, there is an emergency: septic shock threatens because the intestinal bacteria enter the bloodstream. Such a breakthrough does no harm to the Galleria wax moth, which produces a variety of antimicrobial substances that are stored in tiny “canisters” around the intestine. “If the intestine sheds a layer, everything is poured out simultaneously. We were able to show that the number of intestinal bacteria then drops by a factor of 100.”

A Gray Moth as Model

The moth’s immune system uses the same substances to combat harmful germs. “We asked ourselves: If this happens regularly, will resistant germs in the insect develop, similar to the frequent use of antibiotics in hospitals?” says Jens Rolff. Galleria is also interesting for science for another reason, namely because it is comfortably warm in the beehive. “Because the pathogens of the wax moth grow there almost at human body temperature, the gray butterfly is a good infection model for biomedicine.”

The honeybee is being used to investigate the bacilli that cause foul brood, which kills the bee larvae. The pathogens can only infect if the young bee larvae do not yet have a microbiome. The time frame for the infection is extremely small.

It is getting really interesting with the beewolf, a type of wasp that hunts honeybees. In the furrows of its fine antennas, it cultivates fungi that produce antibiotics – up to 45 different substances!

The researchers in Mainz want to investigate how this cocktail affects the microbiome and the pathogens of the beewolf. The fruit fly Drosophila melanogaster has been an important model animal in biology for more than 100 years. However, it is completely unknown which microbes normally live on and in their bodies when they live in the “wild” – in other words in the haze of an apple tree or fruit basket – because when Drosophila is bred for research purposes, its microbiome is shaped by the laboratory environment and what it is fed there.

The researchers are also interested in what happens when the fly is injured. Wounds often occur during mating or because the animal loses a leg, for example, when it has to free itself from a sticky spider web. As with human blood, the hemolymph, or clear insect blood, initiates wound closure. However, wounds are always potential entry points for germs that sit on the surface of the body.

The researchers are also investigating how often fruit flies are injured and which germs penetrate them. Jens Rolff emphasizes that up to now, scientists have underestimated how the impact that wounds have on the fly’s immune system caused it to develop as we know it today.

Over a million species of insects have been described scientifically. That is more than 60 percent of all known animal species at all. This is not surprising because insects play a major role in most ecosystems, for example, as a food base for birds and small mammals. They are irreplaceable for humans as pollinators in food production.

Findings Could Apply to Mammals

Among the insects there are a number of pests that spread to crops, such as the locusts that have been attacking large areas of the countries of East Africa since the beginning of the year. Others, such as the Anopheles mosquito, transmit dangerous diseases to humans or animals. “These are all important reasons to understand insects,” Rolff stresses.

What expectations does Rolff have for the DFG project? He explains, “There are many theories about when and why pathogens become virulent. However, none of them explain how this works in connection with the host’s immune system and its microbiome.”

The researchers use classical biological and molecular biological methods, bioinformatics, and theoretical biology. Mathematical models will help them to transfer what they learn from one species of insect to another, perhaps even on completely different groups of organisms such as vertebrates or even mammals. That is not as improbable as it might seem: the innate immune system was first discovered in insects in the 1970s. In the 1980s, the so-called Toll receptor was isolated, which is crucial for the detection of disease germs by the immune system. Only later was this receptor also found in mice and humans. Homo sapiens also have endogenous antimicrobial proteins.

More than 20 of the antimicrobial proteins similar to those found in insects are currently in clinical development – as potential new antibiotics. The initial hope that pathogens could not develop resistance to them has not been fulfilled. “We were able to show that resistance is very likely, but it develops much more slowly.”

After all, the proteins are very quick “killers” – due to their positive charge, they destabilize the negatively charged cell membrane of the pathogen: they attach themselves and literally eat holes in them. The “enemy” runs out and is killed within a few minutes. A classic antibiotic takes two hours to do this.

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This text originally appeared in German on April 26, 2020, in the Tagesspiegel newspaper supplement published by Freie Universität.