Apex Predator Nematodes and Meso-Predator Bacteria Consume Their Basal Insect Prey through Discrete Stages of Chemical Transformations

Authors: MUCCI, NICCOLAS - University Of Tennessee; JONES, KATARINA - University Of Tennessee; CAO, MENGYI - University Of Wisconsin; WYATT, MICHAEL - University Of Tennessee; FOYE, SHANE - University Of Wisconsin; KAUFFMAN, SARAH - University Of Tennessee; TAUFER, MICHELA - University Of Tennessee; TAKIZAWA, YUKO - Hokkaido University; CHIKARAISHI, YOSHITO - Hokkaido University; Steffan, Shawn; CAMPAGNA, SHAWN - University Of Tennessee; GOODRICH-BLAIR, HEIDI - University Of Tennessee; RICHARDS, GREGORY - University Of Wisconsin

Submitted to: American Society for Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/30/2022
Publication Date: 5/11/2022
Citation: Mucci, N., Jones, K., Cao, M., Wyatt, M., Foye, S., Kauffman, S., Taufer, M., Takizawa, Y., Chikaraishi, Y., Steffan, S.A., Campagna, S., Goodrich-Blair, H., Richards, G.R. 2022. Chemical ecology of a tripartite symbiosis. American Society for Microbiology. 00312-22. https://doi.org/10.1128/msystems.00312-22.
DOI: https://doi.org/10.1128/msystems.00312-22

Interpretive Summary: Many animals rely intimately on microbes to provide access to resources. There has been much recent interest in gut microbes, but there are symbiotic microbes that may be just as important to the animal host. Nematodes are among the most abundant animals on the planet, and thus to understand how the animal host and microbial symbiont co-evolve, it is critical to employ methods that can isolate and quantify these mechanisms. Using carnivorous nematodes, their bacterial symbionts, and their insect hosts, we have examined how these three symbiont interact. We show that the nematodes are analogous to a 'shepherd' that guide their 'flock' (microbes) to the food that the flock feeds upon--insect hosts. When the bacteria have killed the insect, the bacteria feed upon it. Then, the nematodes feed (almost exclusively) upon these bacteria. These are novel findings, and re-define the trophic identities of both the nematodes and their bacterial symbionts.

Technical Abstract: Microbial symbiotic interactions, mediated in part by small molecule signaling and communication, drive physiological processes of many higher order systems. Such exchanges have been revealed through analysis of metabolic “snapshots”, or one time point investigations. Most studies lack the context of how the chemical ecology of the symbiosis changes over the course of the symbionts’ life cycle and association. The tripartite relationship between the nematode host Steinernema carpocapsae, its obligate mutualist bacterium, Xenorhabdus nematophila, and the insects they infect together is a system by which a chemical environment with players exhibiting both antagonistic and mutualistic behaviors can be studied. The nematode infective juveniles (IJs that carry the bacterial symbiont) infect insects and release the bacteria, which will suppress the insect immune response, help kill the insect within 24 hours. The cadaver is a nutrient-rich environment that is exploited by both the nematode and the bacterium until nutrients are depleted, causing the bacteria to colonize a new generation of nematodes that search for more insect prey. The nematode and bacteria mutualists exploit the insect resources for reproduction, but the processes by which the insect biomass is converted, and whether such processes vary as the insect is consumed is largely unknown. Trophic analyses were performed to understand each player’s position in the food chain. Trophic positioning registered S. carpocapsae at 4.37, suggesting the nematodes primarily feed on the bacteria and likely also consume dead nematodes. To gain insights into the metabolism of nematode and bacterium reproduction metabolic profiles of uninfected and nematode-IJ-infected Galleria mellonella were compared. Whole insect samples were taken over the course of the 16-day infection lifecycle. Two hypotheses were tested- : (i) infection will reshape the insect host metabolism before death and (ii) after insect death metabolic profiles will have distinctive groupings depending on early (1 day), middle (2-8 days), and late (10-16 days) time clusters, representing changes in metabolic processes of biomass conversion as reproduction occurs through generations. Multivariate statistics were performed to understand how time point metabolic profiles cluster in relation to each other and to identify groups of metabolites that exhibit similar rates of change. These analyses revealed distinct separation of the time clusters from the uninfected samples, indicating the chemical environment within the cadaver changes over time as nematodes and bacteria reproduce. Metabolic clustering revealed metabolites involved in nucleic acid, amino acid, and lipid metabolism show similar patterns of change to each other. Tricarboxylic acid (TCA) cycle components and proline were significantly affected throughout the infection. Further analysis of trends in metabolic clustering throughout the lifecycle will help inform the chemical ecology of more complex symbioses.