ABSTRACT
INTRODUCTION Traditional identification methods used in bacteriology cannot be routinely applied to rickettsiae because of the few phenotypic characters expressed by these strictly intracellular organisms. As a consequence, “Rickettsia” has long been used as a generic term for many small bacteria that could not be cultivated and were not otherwise identified. However, taxonomic progress made over the last 35 years has deeply modified the definition of “rickettsia.” In particular, the introduction of molecular techniques has revolutionized the study of gene and genome evolution and has allowed new approaches to phylogenetic and taxonomic inferences. As a result of deep taxonomic changes, the term “rickettsia” currently only applies to arthropod-borne bacteria belonging to the genus Rickettsia within the family Rickettsiaceae in the order Rickettsiales, -Proteobacteria. The Rickettsia genus is currently made of 24 recognized species, and also contains several dozens of as-yet uncharacterized strains or tick amplicons. Most of these bacteria are associated with ticks, which are their vectors and reservoirs, but some are vectorized by lice, fleas, or mites. In contrast with louse-and flea-borne rickettsioses, tick-borne rickettsioses have specific geographic distributions, directly depending on the distribution of their vectors. Rickettsia species cause rickettsioses, which are among the oldest known arthropod-borne diseases (1). Currently, 16 rickettsioses are recognized. Among these, several are caused by rickettsiae that were initially isolated from ticks and subsequently considered as nonpathogenic. A priori, it is difficult to predict which rickettsiae are potential human pathogens. It should be considered that rickettsiae found in arthropods capable of biting humans are potential human pathogens.
