ABSTRACT
Pathogenic bacteria must overcome several physiological and immunological challenges to successfully infect even a single type of host, such as a mammal. It is remarkable, then, that bacteria transmitted by blood-feeding arthropods are capable of infecting two very different hosts during their life cycle: an invertebrate (usually an insect or tick) and a mammal. As if this were not enough of a challenge, it is not sufficient that an arthropodborne bacterium successfully infect both vector and host. It must establish a transmissible infection in both; that is, it must infect the vector in such a way as to be transmitted during a blood meal, and it must infect the mammal in a way that allows uptake by a blood-feeding arthropod. This feat of evolution has occurred relatively rarely, but nonetheless arthropod-borne transmission has developed independently in a phylogenetically diverse group of microorganisms, including the rickettsiae, spirochetes in the genus Borrelia, and the Gram-negative bacteria.
