Pathogens are the root cause of diseases. Evolution has equipped them with the ability to evade host immunity through the development of sophisticated mechanisms. While medical intervention can aid the immune system in its fight, it may also result in unintended consequences, such as the emergence of antibiotic-resistant bacteria.

Social intervention is another approach in the fight against pathogens. Social groups, such as ants, utilize collective hygiene and health measures, known as “social immunity”, to prevent the spread of infection within their community. However, the ability of pathogens to respond to this type of group behavior remains unclear.

A recent study conducted by Professor Sylvia Cremer and her team at the Institute of Science and Technology Austria (ISTA) highlights the impact of host-parasite interactions in this regard.

Collaborating with chemical ecologists at the University of Würzburg in Germany, the team studied social ants to investigate how pathogenic fungi react to the social care interventions of their hosts during infections.

The findings show that fungi decrease their chemical signals to evade social immunity.

The research has been published in the journal Nature Ecology & Evolution.

Less Grooming, More spores

“Fungi infect the ants from the body surface and grow inside,” points out lead author Barbara Milutinović, “but nestmates groom off many of the spores before they can even cause internal infection.”

In an experiment, the scientists infected Argentine ants (Linepithema humile) with pathogenic Metarhizium fungi while observing their behavior in the absence or presence of colony members providing care.

Milutinovi says, “We found that the fungi fundamentally changed in response to the ant workers’ caregiving”.

Fungi exposed to grooming nestmate ants produced more spores than those exposed to merely individual ants throughout the course of 10 infection cycles.

“Producing more spores will help the fungus counteract the spore-removal by helping nestmates. Yet, we were surprised to see that the ants showed less grooming against the spores,” adds Sylvia Cremer. “This suggests, that the spores have become more difficult to detect by the ants.”

Fungi change chemically

The researchers collaborated with a chemical ecologist from the University of Würzburg to investigate why ant workers had trouble detecting fungi and to examine potential fungal detection signals.

“The fungi, that adapted to social hosts were perceived less strongly, due to a strong reduction of a fungi-specific compound called ergosterol,” according to Local Professor Thomas Schmitt.

All fungi possess the necessary membrane component ergosterol. The researchers demonstrated that only the fungal component caused vigorous grooming by introducing the ants to it or its slightly different non-fungal vertebrate counterpart.

“This demonstrates that fungal pathogens react to the presence of caregiving ants by reducing their characteristic fungal signals. They are no longer recognized as a disease threat and can escape the social immunity of the colony,” concludes Milutinović.

Results show that social hosts may influence viruses via collective actions.

“It’s fascinating how collective hygiene measures trigger evasion strategies in the pathogen. It would be interesting to see how the ant colony will react in turn. Maybe they become more sensitive in detecting lower and lower fungal cues,” Cremer adds.

Source: 10.1038/s41559-023-01981-6

Image Credit: Sina Metzler, Roland Ferrigato/ISTA