Comparative genome analysis of the α-proteobacteria: Relationships between plant and animal pathogens and host specificity

The phylogenetic group of α-proteobacteria contains bacterial species with a wide variety of lifestyles, including obligate intracellular (Rickettsia), facultative intracellular (Bartonella, Brucella), and extracellular pathogens (Agrobacterium), as well as symbionts of both animals and plants (Wolbachia, Sinorhizobium). With the recent completion of genome sequences of several of these organisms, it is now possible to explore the genetic basis for these biological differences. The newest addition to this small group of complete α-proteobacterial genome sequences is that of Brucella suis, described by Paulsen et al. (1) in this issue of PNAS. B. suis is an intracellular pathogen of swine, where it causes late-term abortions. This infection can be readily transmitted from swine to abattoir workers and farmers via aerosol, thus causing a febrile illness. The efficiency of its transmission via aerosol prompted the military development of B. suis as a biological weapon (2). Although the Brucellae are well known as animal pathogens and are considered to be potential agents of bioterrorism, until recently, most of the research on these organisms had been focused on eradication of brucellosis in cattle and swine and was limited to epidemiology and development of vaccines. As a result, our understanding of the basic biology of this organism is still in its infancy. The completion of the second Brucella genome sequence, therefore, provides a powerful tool for the Brucella research community to address questions on the biology, ecology, and pathogenesis of this group of organisms. Each of these species is adapted to different hosts, yet the genomes of the two Brucella species differ by only 74 genes.