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Insect-specific 'immune priming' affects the evolution of pathogenic bacteria

A research team at the University of Münster has investigated for the first time how the confrontation of bacteria with hosts that have an activated innate immune system affects the evolution of bacterial virulence.

Like all vertebrates, humans have two types of immune memory: the memory of the acquired (adaptive) immune system, which is highly specific to certain pathogens and long-lasting and makes vaccinations possible, and that of the innate immune system—a trained immunity—which reacts quickly but less specifically.

Invertebrates such as insects only have the innate immune system, but they also possess a form of immunization through contact with pathogens (immune priming). Until now, there has been no study on how the confrontation of pathogens with hosts that have such an activated innate immune system affects the evolution of pathogens, and specifically their dangerousness, or virulence.

A research team at the University of Münster led by evolutionary biologist Prof Joachim Kurtz, along with a team headed by Prof Alexander Mellmann from the Institute of Hygiene at the Faculty of Medicine, has now investigated this for the first time through experimental evolution of an insect pathogen (Bacillus thuringiensis tenebrionis) in red flour beetles. One result: after some time of evolution, the pathogen's virulence differed significantly between the different bacterial lines. This greater diversity could accelerate the adaptation of pathogens to their hosts.

In their published in PLOS Pathogens, virulence was measured by how many beetles were killed by infection with the bacteria. On average, it did not change. The fact that the variance in the bacteria increased significantly over generations when the beetles had previously been in contact with bacterial substances could be related to increased activity in certain components of the evolved bacterial genome (prophages and plasmids).

"It is not only of fundamental interest to know under what conditions the virulence of bacteria changes evolutionarily. Our findings are interesting from a medical point of view, for example, if we want to prevent pathogens from becoming more dangerous through evolutionary processes," says lead author Dr. Ana Korša.

Immune priming also plays a practical role in invertebrates, for example, in aquaculture, where invertebrates such as shrimp are bred in large numbers and are susceptible to disease. "In addition, the bacterium we studied and the toxins it produces have long been used for pest control. It would be important to know whether the immune memory of these hosts can evolutionarily alter the virulence of the bacterium," says Korša.

For the experimental evolution of the bacteria, some of the red flour beetles were exposed to bacterial substances (immune priming), while others were not (control group). They were then infected with the bacteria. To allow the bacteria to evolve, they were isolated from the dead beetles and used again for infection. The cycle was repeated several times. The scientists then measured the virulence and other characteristics of the evolved bacteria and examined them genetically.