ABSTRACT
INTRODUCTION In the last decade, we have experienced a major revolution in the biological sciences resulting from a tremendous flux of genome sequence information. Availability of complete microbial genome sequences has greatly benefited our knowledge of microorganisms with regard to their evolutionary history, the metabolic processes they catalyze, as well as their antigenic proteins and virulence factors. Obtaining a whole bacterial sequence is not a trivial undertaking. Therefore, technical progress in sequencing and data processing continue to reduce both the time and expenses involved (1,2). According to the genome project database of the National Center for Biotechnology Information at the National Institutes of Health, over 300 genomes have been completely sequenced and another 573 are in progress. Here, we review insights gained from genome sequences of bacteria of the order Rickettsiales (the genera Rickettsia, Ehrlichia, Anaplasma, Neorickettsia, and Wolbachia) (Table 1, Fig. 1). These bacteria are obligate intracellular parasites of eukaryotes, and many of them are pathogens. Based on their similar lifestyles and parasitic strategies, they were historically considered “Rickettsia-like.” Their phylogenic diversity was highlighted from 16S rRNA gene sequence analysis (14). Because of the intrinsic difficulty in working with these intracellular bacteria and the lack of adequate methods for their genetic manipulation, their biology is still poorly understood. To better grasp the molecular mechanisms underlying their evolution and pathogenicity, genome sequencing efforts have targeted these parasites/pathogens. While obtained genomic data revealed marked similarities between them, each genome exhibited specific features, reflecting a large diversity in their parasitic and infectious strategies. A number of genes emerging from these genomic studies provided a remarkable opportunity to enhance our understanding of these bacteria.
