Immune systems develop silver bullet defenses against bacteria

Antimicrobial peptides (AMPs) are host-encoded immune effectors first identified for their function in infection resistance. In plants and animals, AMPs play a crucial role in determining the composition of the host microbiome. Although multiple studies have revealed that AMPs evolve rapidly, little is understood about the selection factors that drive such evolution.

According to new research, immune systems generate particular genes to battle common bacteria like those in food. Previous hypotheses stated that antimicrobial peptides, a natural antibiotic, broadly function in killing various bacteria.

The latest study in Science examined how bacteria in fruit flies’ food and surroundings alter their immune systems. Dr. Mark Hanson from the Centre for Ecology and Conservation on Exeter’s Penryn Campus in Cornwall said, “We know that an animal’s food and environment determine the bacteria it encounters.”

According to a new study from the Swiss Federal Institute of Technology and the University of Exeter, immune systems create particular genes to battle common bacteria in food. The researchers discovered two peptides that control a single bacterium species that fruit flies encounter. 

The researchers discovered that an animal’s diet and environment influence the bacteria it encounters, which forms its microbiome. The research shows how immune systems react to this, allowing them to regulate common pathogens that could otherwise cause harm. Acetobacter, prevalent in the fruits that fly eat, can injure flies if it exits the gut and enters the bloodstream.

The scientists also discovered signs of “convergent evolution,” which occurs when different species respond similarly to environmental obstacles. The new study found that despite diverging from a common ancestor around 100 million years ago, both fly species acquired a Diptericin B peptide to regulate Acetobacter. Because Acetobacter is no longer prevalent in their environment, closely related fly species that do not feed on fruit have lost their Diptericin B peptides. This evolutionary mechanism could explain why humans are susceptible to some viruses. 

The finding helps us see our immune system from a new perspective and understand why it is built the way it is, which can help us battle infections, especially those that resist antibiotics. 

The findings show a one-sided evolutionary dynamic in which the host modified its immune repertoire to environmental microorganisms, resulting in the rapid evolution of AMP gene families and the formation of “vestigial” immune effectors.

The Swiss National Science Foundation and the Novartis Foundation funded the study.

Journal Reference:

M.A.Hanson, L Grollmus, etal. Ecology-relevant bacteria drive the evolution of host antimicrobial peptides in Drosophila. Science. DOI: 10.1126/science.adg5725

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