Interactions between microbiota and immunity shape pathogen acquisition and fitness in Amblyomma spp. ticks

Besides carrying pathogens, ticks also harbor commensal and mutualistic microorganisms that constitute their microbiota. This microbial community can modulate the tick immune system and influence pathogen acquisition, either facilitating or hindering colonization. Additionally, the microbiota may impact tick fitness. Although the ticks Amblyomma sculptum and Amblyomma aureolatum are important vectors of Rickettsia rickettsii, the causative agent of Brazilian spotted fever, A. sculptum is much less susceptible to infection than A. aureolatum. Intriguingly, while A. aureolatum midgut harbors an abundant microbiota, mostly composed of bacteria of the Francisella genus, A. sculptum presents a markedly reduced bacterial community. In the current study, we quantified the total bacterial load also in the salivary glands and ovaries of adult A. sculptum and A. aureolatum, besides midgut. Across all analyzed organs, bacterial loads were consistently lower in A. sculptum than in A. aureolatum, regardless of whether the ticks had fed on naïve or R. rickettsii-inoculated hosts. High-throughput sequencing of the V3-V4 hypervariable region of the bacterial 16S rRNA gene revealed that Francisella endosymbiont is the dominant taxon in all organs of control A. aureolatum, with the highest relative frequency in the ovaries and the lowest in the midgut. The highest relative frequency of Francisella in the ovaries correlates with the lower susceptibility of this organ to R. rickettsii, suggesting that the endosymbiosis may limit infection. No 16S rRNA gene sequences could be obtained for A. sculptum samples, likely due to their low bacterial content. To investigate the role played by the microbiota on rickettsial acquisition and tick fitness, A. aureolatum engorged females were treated with either tetracycline or ciprofloxacin. Tetracycline treatment significantly reduced bacterial loads and antimicrobial peptide transcript levels in the eggs, and this was followed by a higher acquisition of R. rickettsii by hatched larvae. Additionally, tetracycline negatively impacted tick development, reducing the molt success from the larval to the nymphal stage. These results suggest that maternal microbiota plays a role in shaping offspring immunity, pathogen susceptibility, and tick development. The multifaceted role of tick microbiota in both development and vector competence underscores its potential as a biotechnological resource for developing new strategies to control tick-borne diseases.