Tick microbiomes; are endosymbiotic bacteria mutualists, commensals or hyperparasites?

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Advances in genomics provide exciting opportunities to study the microbiomes (i.e. microbial communities) of arthropods. Ticks, obligate blood-feeding arthropods, transmit the greatest variety of pathogenic agents to domestic animals and humans in North America. These parasitic arthropods are also hosts of non-pathogenic bacterial endosymbionts that are assumed to be beneficial (i.e. mutualists) or of no harm (i.e. commensals) to their tick host; however, this remains to be tested. Recent studies show that a tick’s immune system not only targets pathogenic bacteria but also regulates the numbers of endosymbiotic bacteria. Demonstrating that some non-pathogenic bacteria have a negative effect on their tick host would shift discussion on the functional roles of bacterial endosymbionts to a new paradigm: that is, they are parasites of a parasite (i.e. hyperparasites).