ABSTRACT
Bacteria of the order Rickettsiales (Alphaproteobacteria) are gram-negative, small, rod-shaped, and coccoid, with all described species existing as obligate intracellular parasites of a wide range of eukaryotic organisms (Gillespie et al. 2012b). Before the DNA revolution, bacteria were assigned to Rickettsiales based primarily on chemical composition and morphology. Intraordinal classication employed a taxonomic system (i.e., generic characteristics) based on ve major biological properties: (1) human disease and geographic distribution, (2) natural vertebrate hosts and other animal vectors, (3) experimental infections and serology reactions and cross-reactions, (4) strain cultivation and stability, and (5) energy production and biosynthesis. This pioneering systematic work resulted in a Rickettsiales hierarchy of three families containing nine obligate and facultative intracellular pathogenic genera: (1) Rickettsiaceae: genera Rickettsia, Coxiella, Rochalima, and Ehrlichia; (2) Bartonellaceae: genera Bartonella, Haemobartonella, Eperythrozoon, and Grahamella; and (3) Anaplasmataceae: genus Anaplasma. However, rickettsial classication has been substantially revised since the turn of the millennium, owing to the technological advances in molecular sequence generation and the advent of several new elds of study, including molecular systematics, phylogenomics, and bioinformatics. Contemporary Rickettsiales taxonomy is radically different from the traditional system, with such tremendous changes as the reassignment of the Q-fever agent (Coxiella burnetii) to Gammaproteobacteria and the placement of the causative agents of several human diseases such as endocarditis, trench fever, and cat-scratch disease (Bartonella spp.) to the Order Rhizobiales.
