Investigating symbiont-mediated protection in the eastern subterranean termite (Reticulitermes flavipes Kollar)

Subterranean termites are eusocial pest insects that live in colonies. Termite workers forage to find resources for the colony, encountering many potential pathogens in the soil as they go. To reduce the threat of epizootic events, termites have evolved behavioral mechanisms such as allogrooming, cannibalism, and burial of afflicted individuals. Other physiological responses like the secretion of antimicrobial peptides, up-regulation of defensive enzymes, and circulating hemocyte density all work to impede pathogen infectivity on an individual level. Additionally, in other eusocial and non-social insects, symbiotic microorganisms have been suspected to play a role in immunological function and diseases resistance. Here we investigated the importance of termite symbionts in immunocompetence by evaluating mortality, immune-responsive gene expression, and hemocyte counts following pathogen challenges. In a prior study we found that symbiont removal significantly reduced lignocellulose digestion potential, and here, we hypothesized that termite symbionts mitigate the effects of pathogens by improving host immunocompetence. We challenged with a bacterial pathogen (Serratia marcescens), and the resulting data show a significant increase in host immune response following treatment with kanamycin. Our data suggest that pathogen challenge following the removal of termite gut-symbionts with antibiotics results in a reduction to circulating hemocytes (p<0.0001), higher mortality (p=0.0491), and increased expression of some immune-associated genes (p=0.0341-0.0001). Additionally, we found other immune-associated genes were not affected by symbiont removal, but were up-regulated in response to pathogen challenge (p=0.0106-0.0003).  These data contribute to our understanding of symbiont importance in termite nutritional-immunity by showing a symbiont-mediated protection for termites against pathogens like S. marcescens.