Exploring the potential for actinobacteria as defensive symbionts in fungus-growing termites

In fungus-growing termites, fungi of the subgenus Pseudoxylaria threaten colony health through substrate competition with the termite fungus (Termitomyces). The potential mechanisms with which termites suppress Pseudoxylaria have remained unknown. Here we explore if Actinobacteria potentially play a role as defensive symbionts against Pseudoxylaria in fungus-growing termites. We sampled for Actinobacteria from 30 fungus-growing termite colonies, spanning the three main termite genera and two geographically distant sites. Our isolations yielded 360 Actinobacteria, from which we selected subsets for morphological (288 isolates, grouped in 44 morphotypes) and for 16S rRNA (35 isolates, spanning the majority of morphotypes) characterisation. Actinobacteria were found throughout all sampled nests and colony parts and, phylogenetically, they are interspersed with Actinobacteria from origins other than fungus-growing termites, indicating lack of specificity. Antibiotic-activity screening of 288 isolates against the fungal cultivar and competitor revealed that most of the Actinobacteria-produced molecules with antifungal activity. A more detailed bioassay on 53 isolates, to test the specificity of antibiotics, showed that many Actinobacteria inhibit both Pseudoxylaria and Termitomyces, and that the cultivar fungus generally is more susceptible to inhibition than the competitor. This suggests that either defensive symbionts are not present in the system or that they, if present, represent a subset of the community isolated. If so, the antibiotics must be used in a targeted fashion, being applied to specific areas by the termites. We describe the first discovery of an assembly of antibiotic-producing Actinobacteria occurring in fungus-growing termite nests. However, due to the diversity found, and the lack of both phylogenetic and bioactivity specificity, further work is necessary for a better understanding of the putative role of antibiotic-producing bacteria in the fungus-growing termite mutualistic system.