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 pre-existing 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 rRNA gene sequences - prior to parasite exposure can be informative of the eventual infection outcome. Specifically, higher microbiota OTU diversity and evenness are associated with less resistance. However, the microbial community structure does not differ between infected and non-infected individuals, or between infected individuals of varying susceptibility. This suggests that parasite infection success depends on microbiota composition but subsequent changes occur, although the exact alteration that occur remain elusive. In fact, the bumblebee microbiota is surprisingly unaffected by parasite exposure and infection. Rather, the microbiota-host interactions prior to parasite exposure seems a key mechanism regulating resistance to infection.