Endosymbiont interference and microbial diversity of the Pacific coast tick,Dermacentor occidentalis, in San Diego County, California

Abstract

The Pacific coast tick,Dermacentor occidentalisMarx, is found throughout California and can harbor agents that cause human diseases such as anaplasmosis, ehrlichiosis, tularemia, Rocky Mountain spotted fever and rickettsiosis 364D. Previous studies have demonstrated that nonpathogenic endosymbiotic bacteria can interfere withRickettsiaco-infections in other tick species. We hypothesized that withinD. occidentalisticks, interference may exist between different nonpathogenic endosymbiotic or nonendosymbiotic bacteria and Spotted Fever group Rickettsia (SFGR). Using PCR amplification and sequencing of therompA gene and intergenic region we identified a cohort of SFGR-infected and non-infectedD. occidentalisticks collected from San Diego County. We then amplified a partial segment of the 16S rRNA gene and used next-generation sequencing to elucidate the microbiomes and levels of co-infection in the ticks. The SFGRR. philipiistr. 364D andR. rhipicephaliwere detected in 2.3% and 8.2% of the ticks, respectively, viarompA sequencing. Interestingly, next generation sequencing revealed an inverse relationship between the number ofFrancisella-like endosymbiont (FLE) 16S rRNA sequences andRickettsia16S rRNA sequences within individual ticks that is consistent with partial interference between FLE and SFGR infecting ticks. After excluding theRickettsiaand FLE endosymbionts from the analysis, there was a small but significant difference in microbial community diversity and a pattern of geographic isolation by distance between collection locales. In addition, male ticks had a greater diversity of bacteria than female ticks and ticks that weren’t infected with SFGR had similar microbiomes to canine skin microbiomes. Although experimental studies are required for confirmation, our findings are consistent with the hypothesis that FLEs and, to a lesser extent, other bacteria, interfere with the ability ofD. occidentalisto be infected with certain SFGR. The results also raise interesting possibilities about the effects of putative vertebrate hosts on the tick microbiome.