Enhancement of antimicrobial diversity in situ through relaxed symbiont specificity in an insect/actinomycete partnership

Some insects form symbioses in which actinomycetes provide defense against pathogens by making antimicrobials. The range of chemical strategies employed across these symbioses, and how these strategies relate to insect social behavior and mechanisms of symbiont transmission, remains underexplored. Here, we assess subsocial passalid beetles Odontotaenius disjunctus (known as bessbugs), and their frass (fecal material), as a model insect/actinomycete system. Through chemical and phylogenetic analyses, we found that O. disjunctus associates with an exceptionally wide variety of actinomycetes and antimicrobials. Metabolites detected directly in frass displayed both synergistic and antagonistic inhibition of a fungal entomopathogen, Metarhizium anisopliae, and multiple streptomycete isolates inhibited this pathogen when co-cultivated directly in frass. Together, these findings support a model in which coprophagy as a vertical transmission mechanism leads to relaxed symbiote specificity, resulting in a rich and dynamic repertoire of antimicrobials that insulates O. disjunctus against the evolution of pathogen resistance.