Diversity and structure of the bacterial microbiome of the American dog tick, Dermacentor variabilis, is dominated by the endosymbiont Francisella

Abstract

The American dog tick, Dermacentor variabilis Say, is the principal vector of spotted fever group rickettsiae that cause illness in people. Because the composition and structure of microbial communities in D. variabilis are poorly understood, we conducted high-throughput amplicon sequencing of the 16S rRNA gene to characterize the bacterial communities of adult D. variabilis ticks collected from field populations in North Carolina. Sequence analyses were performed using QIIME 2 with the DADA2 plugin and taxonomic assignments using the Greengenes database. After quality filtering and rarefaction, the bacterial DNA sequences were assigned to 432 operational taxonomic units (OTUs) and were dominated by a single OTU classified as Francisella spp. Subsequent cloning, sequencing and phylogenic analysis of nearly full length 16S rRNA gene fragments revealed that most Francisella were similar to Francisella-like endosymbionts (FLEs) of D. variabilis reported previously. One cloned Francisella, however, was genetically distinct and more related to the FLE of D. occidentalis. The core microbiome of D. variabilis was comprised of Francisella spp., Sphingomonas spp., Delftia spp., and Hymenobacter spp. The taxonomic resolution and detection of less abundant bacterial taxa, including Rickettsia, are discussed. Alpha diversity metrics revealed lower bacterial community diversity in females. Beta-diversity also distinguished the composition of female bacterial communities from those of males. These findings advance current knowledge of the microbial ecology and structure and composition of D. variabilis microbiome and sets the foundation for further studies to determine the influence of microbiota on vector susceptibility to pathogens.