Whole-Genome Analyses of Speciation Events in Pathogenic Brucellae<sup><a href="#fn1" rid="fn1" class=" fn">†</a></sup>

Patrick Chain

Diego Comerci

Marcelo Tolmasky

Frank Larimer

Biosciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94551,^{Instituto de Investigaciones Biotecnologicas, Universidad Nacional de General San Martin (IIB-INTECH-CONICET), Av. Gral. Paz 5445, P.O. Box 30, 1650 San Martin, Buenos Aires, Argentina,}^{Department of Biological Science, California State University, Fullerton, California 92834-6850,}^{Life Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831}⁴

^{Corresponding author. Mailing address: 7000 East Ave. L-441, Livermore, CA 94551. Phone: (925) 422-8002. Fax: (925) 422-2282. E-mail:}vog.lnll@21aicrag.

Received 2005 Jul 13; Revised 2005 Aug 15; Accepted 2005 Sep 19.

Abstract

Despite their high DNA identity and a proposal to group classical Brucella species as biovars of Brucella melitensis, the commonly recognized Brucella species can be distinguished by distinct biochemical and fatty acid characters, as well as by a marked host range (e.g., Brucella suis for swine, B. melitensis for sheep and goats, and Brucella abortus for cattle). Here we present the genome of B. abortus 2308, the virulent prototype biovar 1 strain, and its comparison to the two other human pathogenic Brucella species and to B. abortus field isolate 9-941. The global distribution of pseudogenes, deletions, and insertions supports previous indications that B. abortus and B. melitensis share a common ancestor that diverged from B. suis. With the exception of a dozen genes, the genetic complements of both B. abortus strains are identical, whereas the three species differ in gene content and pseudogenes. The pattern of species-specific gene inactivations affecting transcriptional regulators and outer membrane proteins suggests that these inactivations may play an important role in the establishment of host specificity and may have been a primary driver of speciation in the genus Brucella. Despite being nonmotile, the brucellae contain flagellum gene clusters and display species-specific flagellar gene inactivations, which lead to the putative generation of different versions of flagellum-derived structures and may contribute to differences in host specificity and virulence. Metabolic changes such as the lack of complete metabolic pathways for the synthesis of numerous compounds (e.g., glycogen, biotin, NAD, and choline) are consistent with adaptation of brucellae to an intracellular life-style.

Abstract

Brucellosis is a major infectious disease affecting animals and humans (11). Several Brucella species, such as Brucella abortus, Brucella melitensis, and Brucella suis, have been isolated from a variety of animals. Of these, B. abortus, the causative agent of bovine brucellosis, is the most widespread (11). All three Brucella species cause a severe human disease characterized in its acute phase by undulant fever and in its chronic phase by localization of the pathogen and damage of different organs (11, 38). If localized in the brain or the heart, this can result in fatal meningitis or fatal endocarditis, respectively. Brucella infection is treated with a combination of antibiotics; however, in its chronic phase, eradication is difficult since Brucella spp. are intracellular pathogens, which puts them out of reach of humoral immunity and several antibiotics (25, 38). The lack of a safe and efficacious human vaccine underscores the importance of understanding the biology of brucellosis to develop human vaccines and effective therapeutic agents. Furthermore, all three Brucella species are listed as potential bioweapons by the Centers for Disease Control and Prevention (22, 26). This is due to the highly infectious nature of all three species, the fact that they can be readily aerosolized, and the fact that an outbreak would be difficult to detect because the initial symptoms are easily confused with those of influenza.

Brucella spp. belong to a monophyletic branch of the alpha 2 subgroup of proteobacteria, whose members share the ability to engage in intimate or sometimes intracellular associations with eukaryotic cells (48). However, little is known regarding the biological basis of Brucella sp. host specificity or about the mechanisms involved in the intracellular multiplication and persistence of members of this group. The availability of the B. abortus strain 2308 sequence reported here, together with the sequences of B. suis, B. melitensis, and B. abortus strain 9-941 (12, 18, 35), enabled us to perform a comprehensive examination and comparison of the gene composition, mutations, structural arrangement, and other characteristics of these genomes. Detailed comparisons have confirmed the B. abortus deletions identified with B. melitensis-derived microarrays (37) and have uncovered a large number of additional differences in B. abortus, as well as in B. melitensis and B. suis. Overall, our analyses confirm the striking similarities that exist among the three species and reveal a number of important, though subtle, features that may be important in host specificity and the adaptation of these organisms to an intracellular life-style.

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Acknowledgments

We thank Warren Regala, Brian Souza, and Kevin Melissare for technical contributions.

This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under contract W-7405-ENG-48 and Oak Ridge National Laboratory under contract DE-AC05-00OR22725, the California State University Program for Education and Biotechnology, and the Agencia Nacional de Promoción Científica y Tecnológica, Argentina (PICT 01-11882 and 019-194).

Acknowledgments

Notes

Editor: D. L. Burns

Notes

Editor: D. L. Burns

Footnotes

^{Supplemental material for this article may be found at}http://iai.asm.org/.

Footnotes

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