In nature, hosts from a variety of taxa are commonly infected by a variety of parasites and other symbionts that can facilitate and antagonize the growth and transmission of other infracommunity members. The host can influence these interactions through both top-down and bottom-up processes such as immunity and resource allocation, respectively. However, there is still a gap in knowledge concerning the relative contributions of these processes on infection outcomes and transmission of each parasite. To address this problem, we inoculated flour beetles (Tribolium castaneum) that received low quality, normal, or protein supplemented food with a naturally occurring protozoan eugregarine parasite or a control treatment, and then infected these beetles with an LD50 dose of the entomopathogenic bacterium Bacillus thuringiensis (Bt). We measured bacterial growth dynamics over time, host mortality, host immune gene expression, and bacterial load at time of death.
Results/Conclusions
Individuals that were co-infected with gregarines experienced reduced mortality and reduced bacterial burdens after infection with Bt. Resource supplementation resulted in increased bacterial burden at the time of death in both gregarine treatments but did not significantly affect mortality, suggesting that bottom-up processes regulate bacterial growth dynamics and transmission-stage propagule accumulation. However, eugregarine co-infection stimulated increased production of antimicrobial peptides and may explain increased variance in co-infected bacterial burdens relative to individuals infected only with Bt. Thus, our results suggest that bottom-up processes are important for the dynamics of parasite infra-community interactions in a natural system, while top-down processes may contribute heterogeneity that is predicted to reduce transmission of each parasite.