Three phylogenetically distinct and culturable diazotrophs are perennial symbionts of leaf-cutting ants

What shapes a microbiome? Differences in bacterial communities associated with helminth-amphipod interactions

The fast technological advances of molecular tools have enabled us to uncover a new dimension hidden within parasites and their hosts: their microbiomes. Increasingly, parasitologists characterise host microbiome changes in the face of parasitic infections, revealing the potential of these microscopic fast-evolving entities to influence host-parasite interactions. However, most of the changes in host microbiomes seem to depend on the host and parasite species in question. Furthermore, we should understand the relative role of parasitic infections as microbiome modulators when compared with other microbiome-impacting factors (e.g., host size, age, sex). Here, we characterised the microbiome of a single intermediate host species infected by two parasites belonging to different phyla: the acanthocephalan Plagiorhynchus allisonae and a dilepidid cestode, both infecting Transorchestia serrulata amphipods collected simultaneously from the same locality. We used the v4 hypervariable region of the 16S rRNA prokaryotic gene to identify the hemolymph bacterial community of uninfected, acanthocephalan-infected, and cestode-infected amphipods, as well as the bacteria in the amphipods' immediate environment and in the parasites infecting them. Our results show that parasitic infections were more strongly associated with differences in host bacterial community richness than amphipod size, presence of amphipod eggs in female amphipods, and even parasite load. Amphipods infected by acanthocephalans had the most divergent bacterial community, with a marked decrease in alpha diversity compared with cestode-infected and uninfected hosts. In accordance with the species-specific nature of microbiome changes in parasitic infections, we found unique microbial taxa associating with hosts infected by each parasite species, as well as taxa only shared between a parasite species and their infected hosts. However, there were some bacterial taxa detected in all parasitised amphipods (regardless of the parasite species), but not in uninfected amphipods or the environment. We propose that shared bacteria associated with all hosts parasitised by distantly related helminths may be important either in helping host defences or parasites' success, and could thus interact with host-parasite evolution.

Observation of the Antimicrobial Activities of Two Actinomycetes in the Harvester Ant Messor orientalis

Simple Summary

Observations in the animal room have shown that the seeds stored by harvester ants, although in a damp environment, are less likely to mold. It was hypothesized that harvester ants may use actinomycetes to protect their seed stores, given that leafcutter ants use actinomycetes as producers of defensive substances. Two actinomycetes were isolated from the harvester ant Messor orientalis. The fermentation broth of the actinomycetes showed significant inhibitory effects on the three indicator fungi. Coculture experiments supported the observed inhibitory effects. The antifungal activities of actinomycetes in harvester ants were revealed. This research provides a significant theoretical reference for the abovementioned hypothesis and for the potential agricultural applications of these actinomycetes for multiple crops.

Abstract

Observations have shown that seeds collected by harvester ants are less likely to mold. Based on evolutionary analysis and other research, it was hypothesized that harvester ants could apply actinomycetes to protect seeds, similar to the protection of mutualistic fungi by leafcutter ants. Two actinomycetes were successfully isolated from the harvester ant Messor orientalis. The taxonomic status of the actinomycetes was determined by 16S rRNA sequence analysis and biochemical experimental observations. Their inhibitory effects on plant pathogens were measured. One of the bacteria was identified as Brachybacterium phenoliresistens and denoted as B. phenoliresistens MO. The other belonged to the genus Microbacterium. It was named Microbacterium sp. Growth rate determination and coculture experiments were performed to explore the inhibitory effect of actinomycetes on indicator plant pathogens. The inhibition rates of the actinomycetes toward Peronophythora litchii and Rhizoctonia solani were 100% in media containing 30% or more fermentation broth, and they also showed an inhibitory effect on Colletotrichum siamense. The coculture experiment supported this result by showing that the growth of P. litchii and R. solani was inhibited in the presence of actinomycetes. Therefore, the results of this study show the agricultural application potential of these bacteria and may provide a reference for research on the symbiosis of harvester ants with actinomycetes.

Trachymyrmex septentrionalis Ant Microbiome Assembly Is Unique to Individual Colonies and Castes

Within social insect colonies, microbiomes often differ between castes due to their different functional roles and between colony locations. Trachymyrmex septentrionalis fungus-growing ants form colonies throughout the eastern United States and northern Mexico that include workers, female and male alates (unmated reproductive castes), larvae, and pupae. How T. septentrionalis microbiomes vary across this geographic range and between castes is unknown. Our sampling of individual ants from colonies across the eastern United States revealed a conserved T. septentrionalis worker ant microbiome and revealed that worker ant microbiomes are more conserved within colonies than between them. A deeper sampling of individual ants from two colonies that included all available castes (pupae, larvae, workers, and female and male alates), from both before and after adaptation to controlled laboratory conditions, revealed that ant microbiomes from each colony, caste, and rearing condition were typically conserved within but not between each sampling category. Tenericute bacterial symbionts were especially abundant in these ant microbiomes and varied widely in abundance between sampling categories. This study demonstrates how individual insect colonies primarily drive the composition of their microbiomes and shows that these microbiomes are further modified by developmental differences between insect castes and the different environmental conditions experienced by each colony.

IMPORTANCE This study investigates microbiome assembly in the fungus-growing ant Trachymyrmex septentrionalis, showing how colony, caste, and lab adaptation influence the microbiome and revealing unique patterns of mollicute symbiont abundance. We find that ant microbiomes differ strongly between colonies but less so within colonies. Microbiomes of different castes and following lab adaptation also differ in a colony-specific manner. This study advances our understanding of the nature of individuality in social insect microbiomes and cautions against the common practice of only sampling a limited number of populations to understand microbiome diversity and function.