External immunity in ant societies: sociality and colony size do not predict investment in antimicrobials

Abstract

Social insects live in dense groups with a high probability of disease transmission and have therefore faced strong pressures to develop defences against pathogens. For this reason, social insects have been hypothesized to invest in antimicrobial secretions as a mechanism of external immunity to prevent the spread of disease. However, empirical studies linking the evolution of sociality with increased investment in antimicrobials have been relatively few. Here we quantify the strength of antimicrobial secretions among 20 ant species that cover a broad spectrum of ant diversity and colony sizes. We extracted external compounds from ant workers to test whether they inhibited the growth of the bacterium Staphylococcus epidermidis. Because all ant species are highly social, we predicted that all species would exhibit some antimicrobial activity and that species that form the largest colonies would exhibit the strongest antimicrobial response. Our comparative approach revealed that strong surface antimicrobials are common to particular ant clades, but 40% of species exhibited no antimicrobial activity at all. We also found no correlation between antimicrobial activity and colony size. Rather than relying on antimicrobial secretions as external immunity to control pathogen spread, many ant species have probably developed alternative strategies to defend against disease pressure.

Azteca ants maintain unique microbiomes across functionally distinct nest chambers

“…Given that animal pathogens were common, though not dominant, where ants were present, we do not believe ants themselves were exerting strong suppression outside of the nursery. However, ants are known to produce a range of antimicrobial compounds that could be used to control pathogens inside the nest [24,68]. Additional studies that assess whether these putative animal pathogens affect Azteca fitness are needed to confirm whether their suppression of animal pathogens is evolutionarily advantageous or simply a by-product of ant activity.…”

Genomic analysis of ant domatia-associated melanized fungi (Chaetothyriales, Ascomycota)

“…The expansion of gene families has been associated with the success of free-living black yeasts in colonizing extreme habitats. Although the environment of the domatium occupied by black yeasts can be harsh and toxic because of the antimicrobial chemicals produced by ants (Fernández-Marín et al 2006Penick et al 2018), the chemical stress induced is predictable and the number of compounds relatively low. This could explain the lower number of isoforms for specific gene families in Chaetothyriales specialized in colonizing ant-occupied domatia.…”

Drugs from bugs

“…Penick and co-researchers at the universities of Arizona State, North Carolina State, Campbell, Pennsylvania State and Copenhagen set out to produce an assay to change that. 3 The team tested the antimicrobial properties associated with 20 ant species. They used a solvent to remove all of the substances on the surface of each ant's body, then introduced the resulting solution to a bacterial slurry.…”