Bacterial microbiomes and antifungal effect of Danish wood ant (Formica polyctena) populations

Ants, including wood ants (Formica polyctena), can control arthropod pests, and recent research has highlighted their potential against plant pathogens. The latter effect is likely aided by antimicrobial ant-associated microbes. Here, we investigated the bacterial microbiome of wood ants (F. polyctena), and their nests across five sites in Denmark. We assessed the ants' deposition of antimicrobial microbes with inhibitory effects against the economically important plant-pathogenic fungi, Monilinia fructigena, Botrytis cinerea, and Fusarium graminearum, and hypothesized that deposited microbes are part of a specific, ant-surface-associated microbiome. To assess this, we sampled ant legs and bodies separately, assuming that the bacterial communities of the legs would be distinct from the body and nest microbiomes. Ants deposited antimicrobial microorganisms within 10 s of walking on agar plates, and a 24-h exposure to ants led to dense microbial growth on the agar plates. We isolated seven bacterial and seven fungal antimicrobial strains, with six bacterial isolates matching 16S rRNA amplicon sequences from ant microbiomes across all sites. These sequences were more abundant and prevalent on ant bodies and legs than on nests, suggesting that isolates originated from the ant-surface microbiome. Ant bacterial microbiomes were similar across the five sites, heavily dominated by the endosymbiont Wolbachia, and differed significantly between bodies, legs, and nests, indicating selective acquisition of the surface microbiome instead of random acquisition from the environment. These findings suggest that wood ants have a distinct leg microbiome and are consistently associated with antifungal microorganisms with potential against plant-pathogenic fungi.

This study constitutes the first comprehensive study of the bacterial microbiomes of wood ants (Formica polyctena) and their nests, providing valuable insights into the ants' association with antimicrobial microbes and the composition and compartmentalization of their microbiomes. Our results highlight that wood ants are generally associated with antimicrobial microbes that are effective against commercially important plant-pathogenic fungi, emphasizing their potential as biocontrol agents in agriculture. Additionally, the results show that the bacterial microbiome of wood ants is highly dominated by the endosymbiont Wolbachia, while bacterial microbiomes of wood ant legs, bodies, and nests were all significantly different. These results suggest that the wood ant microbiome is selectively acquired, rather than randomly acquired from the ants' environment. In conclusion, the findings of this study provide new insights into the wood ant microbiome and emphasize the potential of using ants and their associated microbes as biocontrol agents against plant-pathogenic fungi.