Bacterial Associates of a Desert Specialist Fungus-Growing Ant Antagonize Competitors with a Nocamycin Analog.

Fungus-growing ants are defended by antibiotic-producing bacterial symbionts in the genus Pseudonocardia. Nutrients provisioned by the ants support these symbionts but also invite colonization and competition from other bacteria. As an arena for chemically mediated bacterial competition, this niche offers a window into ecological antibiotic function with well-defined competing organisms. From multiple colonies of the desert specialist ant Trachymyrmex smithi, we isolated Amycolatopsis bacteria that inhibit the growth of Pseudonocardia symbionts under laboratory conditions. Using bioassay-guided fractionation, we discovered a novel analog of the antibiotic nocamycin that is responsible for this antagonism. We identified the biosynthetic gene cluster for this antibiotic, which has a suite of oxidative enzymes consistent with this molecule's more extensive oxidative tailoring relative to similar tetramic acid antibiotics. High genetic similarity to globally distributed soil Amycolatopsis isolates suggest that this ant-derived Amycolatopsis strain may be an opportunistic soil strain whose antibiotic production allows for competition in this specialized niche. This nocamycin analog adds to the catalog of novel bioactive molecules isolated from bacterial associates of fungus-growing ants, and its activity against ant symbionts represents, to our knowledge, the first putative ecological function for the widely distributed enoyl tetramic acid family of antibiotics.

Thiopeptide Defense by an Ant's Bacterial Symbiont.

Fungus-growing ants and their microbial symbionts have emerged as a model system for understanding antibiotic deployment in an ecological context. Here we establish that bacterial symbionts of the ant Trachymyrmex septentrionalis antagonize their most likely competitors, other strains of ant-associated bacteria, using the thiopeptide antibiotic GE37468. Genomic analysis suggests that these symbionts acquired the GE37468 gene cluster from soil bacteria. This antibiotic, with known activity against human pathogens, was previously identified in a biochemical screen but had no known ecological role. GE37468's host-associated defense role in this insect niche intriguingly parallels the function of similar thiopeptides in the human microbiome.