Tick microbial associations at the crossroad of horizontal and vertical transmission pathways

Background: Microbial communities can affect disease risk by interfering with the transmission or maintenance of pathogens in blood-feeding arthropods. Here, we investigated whether bacterial communities vary between Ixodes ricinus nymphs, which are (un)infected with horizontally-transmitted human pathogens. Methods: : Ticks from eight forest sites were tested for the presence of Borrelia burgdorferi sensu lato, Babesia spp., Anaplasma phagocytophilum , and Neoehrlichia mikurensis by qPCR, pooled, and their microbiomes were determined by 16S rRNA amplicon sequencing. Tick bacterial communities clustered poorly by pathogen infection status but better by geography. As a second approach, we analysed variation in tick microorganism community structure (in terms of species co-infection) across space using hierarchical modelling of species communities. For that, we analysed almost 14,000 nymphs, which were tested with targeted qPCR for the presence of horizontally-transmitted pathogens B. burgdorferi s.l., A. phagocytophilum , and N. mikurensis , and the vertically-transmitted tick symbionts Rickettsia helvetica , Rickettsiella spp., Spiroplasma ixodetis and Midichloria mitochondrii. Results: : All microorganisms but Rickettsiella spp. had either significant negative ( R. helvetica and A. phagocytophilum ) or positive ( S. ixodetis , N. mikurensis , and B. burgdorferi s.l.) associations with M. mitochondrii . Two tick symbionts, R. helvetica and S. ixodetis , were negatively associated. As expected, both B. burgdorferi s.l. and N. mikurensis had a significant positive association with each other and a negative association with A. phagocytophilum . Although these few specific associations do not appear to affect much the entire microbiome composition, they can still be relevant for tick-borne pathogen dynamics. Conclusions: : Based on our results, we propose that M. mitochondrii alters the propensity of ticks to acquire or maintain horizontally-acquired pathogens. The underlying mechanisms for some of these remarkable interactions are discussed and merit further investigation.