High Gut Microbiota Diversity Provides Lower Resistance against Infection by an Intestinal Parasite in Bumblebees

The microbiome, especially the gut flora, is known to affect the interaction between parasites and their hosts. In this context, a parasitic infection can be viewed as an invasion into the preexisting microbial ecological community. Hence, in addition to the intrinsic defense mechanisms of the host itself, infection success depends on the colonization resistance of the microbiota. In the bumblebee Bombus terrestris, the microbiota provides resistance to the intestinal parasite Crithidia bombi, yet which properties actually provide protection remains largely unknown. Here, we show that the community structure of the gut microbiota-in terms of bacterial operational taxonomic units (OTUs) of 16S ribosomal RNA gene sequences-before parasite exposure can be informative of the eventual infection outcome. Specifically, higher microbiota OTU diversity is associated with less resistance. However, the microbial community structure does not differ between infected and noninfected individuals or between infected individuals of varying susceptibility. This suggests that parasite infection success depends on the microbiota composition but that subsequent changes occur, although the exact alteration that occurs remains elusive. In fact, the bumblebee microbiota is surprisingly unaffected by parasite exposure and infection. Rather, the microbiota-host interaction before parasite exposure seems to be a key mechanism regulating resistance to infection.