Abstract
Ticks are among the most important vectors of pathogens affecting humans and other animals worldwide. They do not only carry pathogens however, as a diverse group of commensal and symbiotic microorganisms are also present in ticks. Unlike pathogens, their biology and their effect on ticks remain largely unexplored, and are in fact often neglected. Nonetheless, they can confer multiple detrimental, neutral, or beneficial effects to their tick hosts, and can play various roles in fitness, nutritional adaptation, development, reproduction, defense against environmental stress, and immunity. Non-pathogenic microorganisms may also play a role in driving transmission of tick-borne pathogens (TBP), with many potential implications for both human and animal health. In addition, the genetic proximity of some pathogens to mutualistic symbionts hosted by ticks is evident when studying phylogenies of several bacterial genera. The best examples are found within members of the Rickettsia, Francisella, and Coxiella genera: while in medical and veterinary research these bacteria are traditionally recognized as highly virulent vertebrate pathogens, it is now clear to evolutionary ecologists that many (if not most) Coxiella, Francisella, and Rickettsia bacteria are actually non-pathogenic microorganisms exhibiting alternative lifestyles as mutualistic ticks symbionts. Consequently, ticks represent a compelling yet challenging system in which to study microbiomes and microbial interactions, and to investigate the composition, functional, and ecological implications of bacterial communities. Ultimately, deciphering the relationships between tick microorganisms as well as tick symbiont interactions will garner invaluable information, which may aid in the future development of arthropod pest and vector-borne pathogen transmission control strategies.
Highlights
Over the last few decades, considerable research efforts have focused on the diversity, distribution, and impact of tick-borne pathogens (TBP)
Extensive literature studies have made it clear that TBP are not alone: an appreciable range of diverse non-pathogenic microorganisms has been detected in almost all tick species examined so far
It is vital to establish the nature of the interactions between non-pathogenic microorganisms, their tick hosts, and co-infecting TBP
Summary
Over the last few decades, considerable research efforts have focused on the diversity, distribution, and impact of tick-borne pathogens (TBP). Formal testing with nutritional and physiological experiments is required to validate their role as alternative obligate symbionts, recent bacterial genome data suggest that these bacteria have highly-evolved adaptive mechanisms enabling tick survival Their genomes encode functions suggesting that they have—at least partially as for Coxiella-LE—a genetic capability for de novo B vitamin synthesis. Worthy of note here is that laboratory findings directly corroborate the existence of beneficial Rickettsia symbionts since they exert a significant effect on larval motility of A. americanum, Dermacentor variabilis, and I. scapularis ticks (Kagemann and Clay, 2013) Overall, these maternally-inherited symbionts are of ecological and evolutionary importance to the tick species they infect, and potentially mediate tick adaptation to hematophagy.
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