Relationships Between Plant and Animal Pathogens and Host Specificity

Commentary

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

Rene´ e M. Tsolis*

Department of Medical Microbiology and Immunology, Texas A&M University System Health Science Center, College Station, TX 77843

he phylogenetic group of ␣-proteobac- determinant. Bacterial surface proteins, Brucella suggests that despite their meta- Tteria contains bacterial species with a such as K88 pili of enteropathogenic Esch- bolic capability to grow outside the host, wide variety of lifestyles, including obligate erichia coli or internalin of Listeria monocy- these pathogens spend most of their time in intracellular (Rickettsia), facultative intra- togenes, have been shown to bind specifically an intracellular location and persist only cellular (Bartonella, Brucella), and extracel- to tissues of their preferred hosts (8, 9). transiently in the environment between lular pathogens (Agrobacterium), as well as Accordingly, closely related bacterial spe- hosts. The success of programs in the U.S. symbionts of both animals and plants (Wol- cies that lack these binding proteins are and other countries for eradication of bachia, Sinorhizobium). With the recent unable to cause disease (10). These findings bovine brucellosis also argues against completion of genome sequences of several suggest that bacterial host range factors can prolonged environmental persistence of of these organisms, it is now possible to be identified by comparing the genomes of Brucella. explore the genetic basis for these biological closely related pathogens that differ in their The small number of genetic islands differences. The newest addition to this host range to find the genes unique to each in the Brucella species makes them an at- small group of complete ␣-proteobacterial pathogen. Comparison of the genomes of tractive model for examining the contribu- genome sequences is that of Brucella suis, B. suis and B. meliten- tion of their unique described by Paulsen et al. (1) in this issue of sis yielded a surpris- genes to host speci- ing finding: whereas ficity. Which host– PNAS. B. suis is an intracellular pathogen of Each of these species is adapted to COMMENTARY swine, where it causes late-term abortions. each of these species pathogen interac- This infection can be readily transmitted is adapted to differ- different hosts, yet the genomes of tions might be ent hosts, the ge- determinants of host from swine to abattoir workers and farmers the two Brucella species differ by via aerosol, thus causing a febrile illness. nomes of the two range? The patho- only 74 genes. The efficiency of its transmission via aerosol Brucella species dif- genic Brucella spe- prompted the military development of B. fer by only 74 genes cies are known to suis as a biological weapon (2). Although the (1, 11). B. suis con- cause systemic infec- Brucellae are well known as animal patho- tains 42 unique genes located in 22 chro- tion in a variety of animal hosts, including gens and are considered to be potential mosomal regions (designated genetic is- humans, where the bacteria traverse epithe- agents of bioterrorism, until recently, most lands), whereas B. melitensis contains 32 lial barriers and localize to the reticuloen- of the research on these organisms had been unique genes on 11 islands (1). This number dothelial system to cause infection. in vitro Brucella focused on eradication of brucellosis in cat- is quite small when compared with the dif- Similarly, , spp. are able to survive in macrophages from different spe- tle and swine and was limited to epidemi- ferences found between pairs of host- cies, including mice and humans (34–37). In ology and development of vaccines. As a adapted Xanthomonas or Salmonella species contrast, the ability of Brucella species to result, our understanding of the basic biol- (Fig. 1; refs. 12 and 13). For reference, the cause abortion is restricted to one or a few ogy of this organism is still in its infancy. The Salmonella enterica serotype Typhi genome hosts, suggesting that factors that contribute completion of the second Brucella genome contains 601 genes (on 82 genetic islands) to tropism and growth in the placenta may sequence, therefore, provides a powerful that are absent from S. enterica serotype determine whether a particular Brucella tool for the Brucella research community to Typhimurium, whereas Salmonella typhi- species can cause abortion in its pregnant address questions on the biology, ecology, murium contains 479 genes (on 80 genetic host. For example, interspecies differences and pathogenesis of this group of organisms. islands) that are unique relative to Salmo- in morphology and function of the placenta, Although B. suis has been isolated from nella typhi (13). In fact, the proportion of including nutrient transport functions and cattle, it has not been found to cause abor- genes unique to the B. melitensis or B. suis maternal recognition of pregnancy, have tion in this host as it does in swine (3–6). genomes is much closer to that of closely been documented (17–19). Thus, bacterial Brucella melitensis related obligate intracellular bacteria, such , on the other hand, has genes involved in host restricted disease as different Buchnera or different Chla- been reported to cause abortion in cattle as phenotypes may encode factors that specif- mydia species (Fig. 1; refs. 14 and 15). It has well as in sheep and goats (7). A question ically enhance localization to or rapid been suggested that the low rate of genetic that has interested researchers in the infec- growth within the placental cells of a tious disease field for years is, what is the exchange among obligate intracellular bac- genetic basis for host specificity? Some stud- teria can be attributed to their isolated lo- ies have shown that the initial interaction of cation within cells of their hosts (16). The See companion article on page 13148. bacteria with host tissues is an important paucity of genetic exchange observed in *E-mail: [email protected].

www.pnas.org͞cgi͞doi͞10.1073͞pnas.212508599 PNAS ͉ October 1, 2002 ͉ vol. 99 ͉ no. 20 ͉ 12503–12505 Downloaded by guest on September 23, 2021 Cyclic ␤-1–2-glucan, a polysaccharide pro- duced by S. meliloti, A. tumefaciens, and B. abortus also contributes in these three species to host–pathogen interactions, as mu- tants are impaired in their persistence in the host (24–26). Similarly, a cytoplasmic membrane transporter, BacA, was shown to be required for both symbiosis of S. meliloti with alfalfa and survival of B. abortus within the macrophage (27). More recently, a type IV secretion with homology to the A. tumefaciens virB genes was identified in B. suis and B. abortus (28, 29). The virB genes are essential for tumor formation on plants by A. tumefaciens, and similarly, the B. abortus virB genes are required for infection in animals (28, 30). Although the precise functions of all of these shared proteins in virulence remain to be elucidated, these findings suggest that animal pathogens, plant symbionts, and animal pathogens all use common mechanisms for interactions with their hosts, and that these may have been adapted to respond to signals in the plant and animal environments. Unlike A. tumefaciens, M. loti, and S. meliloti, B. suis does not contain any extra- chromosomal DNA. However, B. suis chromosome 2 contains numerous homologs of Fig. 1. Results of comparative genomic analyses between closely related bacterial species that differ in genes located on the linear chromosome of pathogenicity or host preference. Each bar represents the number of unique genes relative to its A. tumefaciens or on plasmids of S. meliloti. companion genome (top vs. bottom bars) as a percentage of all ORFs in the genome. The number above These include the virB genes, encoding a each bar indicates the total number of unique genes in each genome (1, 10, 12, 13, 15, 33). *, excludes type IV secretion system and conjugation genes located on one prophage in Listeria monocytogenes and on ﬁve prophages in Listeria innocua. genes trbL-traI (1, 29). Together, these findings suggest that parts of chromosome 2 may have been originally ac- particular host. Several of the genes unique sequence data, to design experimental quired from a plasmid. The recent iden- to B. suis encode putative transport systems approaches aimed at assessing the contribu- tification of the closest sequenced or outer membrane proteins that could con- tion of point mutations in coding genes to homologs to date of the Brucella virB tribute to these functions (1). host specificity of the Brucellae (1). genes on conjugative plasmids from In addition to unique genes, point Comparative genome analysis between B. microbes of the rhizosphere of wheat mutations may contribute to the differences suis and members of the family Rhizobi- and alfalfa lends further support to this in host specificity between B. suis and aceae, which includes Agrobacterium tume- idea (31, 32). B. melitensis. These point mutations may faciens, Sinorhizobium meliloti, and Mesorhi- In analyzing a vast amount of compara- lead to truncations or frameshifts in genes zobium loti, revealed similarities in tive data, Paulsen et al. (1) have managed to that may play a role in host–pathogen inter- metabolic capability and genome structure give us an overview of the complex relation- actions. Finally, the role of point mutations (1). This result extends and confirms results ships between the genomes of the ␣-pro- that change the amino acid sequence of a from several groups that have identified teobacterial species and how we can use the protein should also be considered, as it has virulence factors in Brucella, whose coun- genomic data to gain insight into the ecol- been shown that a single amino acid change terparts in S. meliloti or A. tumefaciens are ogy and biology of a poorly characterized in the E. coli FimH adhesin leads to loss of required for endosymbiosis or pathogenesis organism. The completion of genome se- collagen-binding activity (20). Some of the in plants. For example, a two-component quences of a third Brucella species, B. abor- variable surface proteins identified in the regulatory system, BvrR͞BvrS, has been tus, and its close phylogenetic relative comparative genome analysis of B. suis and shown to be required for intracellular rep- Ochrobactrum anthropi, an opportunistic B. melitensis, therefore, may be additional lication of Brucella abortus (21). Its homolog pathogen, will allow researchers to extend candidates for host-specific virulence fac- in Agrobacterium, ChvG͞ChvI, is required the comparative genomic analysis presented tors. With only 7,301 single nucleotide poly- for tumor formation in plants, and the S. here to better understand the contribution morphisms identified between the B. suis meliloti homolog ExoS͞ChvI regulates pro- of genomic differences to the diverse inter- and the B. melitensis genomes, it should duction of succinoglycan, which is crucial for actions of the different ␣-proteobacterial also be possible, by using the genome the development of endosymbiosis (22, 23). species with their hosts.

1. Paulsen, I. T., Seshadri, R., Nelson, K. E., Eisen, Sidell, F., Takafuji, E. & Franz, D. (Office of the Sur- 5. Washko, F., Hutchings, L. M. & Donham, C. R. J. A., Heidelberg, J. F., Read, T. D., Dodson, R. J., geon General, Washington, DC), pp. 513–521. (1948) Am. J. Vet. Res. 8, 342–352. Umayan, L., Brinkac, L. M., Beanan, M. J., et al. 3. Ewalt, D. R., Payeur, J. B., Rhyan, J. C. 6. Washko, F. & Hutchings, L. (1951) Am. J. Vet. Res. (2002) Proc. Natl. Acad. Sci. USA 99, 13148– & Geer, P. L. (1997) J. Vet. Diagn. Invest. 9, 12, 165–174. 13153. 417–420. 7. Verger, J. M., Garin-Bastuji, B., Grayon, M. & 2. Hoover, D. & Friedlander, A. (1997) in Medical 4. Norton, J. H. & Thomas, A. D. (1979) Aust. Vet. Mahe, A. M. (1989) Ann. Rech. Vet. 20, Aspects of Chemical and Biological Warfare, eds. J. 55, 525–527. 93–102.

12504 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.212508599 Tsolis Downloaded by guest on September 23, 2021 8. Moon, H. W., Nagy, B., Isaacson, R. E. & Orskov, 16. Read, T. D., Brunham, R. C., Shen, C., Gill, S. R., 28. Sieira, R., Comerci, D. J., Sanchez, D. O. I. (1977) Infect. Immun. 15, 614–620. Heidelberg, J. F., White, O., Hickey, E. K., Peter- & Ugalde, R. A. (2000) J. Bacteriol. 182, 9. Lecuit, M., Vandormael-Pournin, S., Lefort, J., son, J., Utterback, T., Berry, K., et al. (2000) 4849–4855. Huerre, M., Gounon, P., Dupuy, C., Babinet, C. & Nucleic Acids Res. 28, 1397–1406. 29. O’Callaghan, D., Cazevieille, C., Allardet- Cossart, P. (2001) Science 292, 1722–1725. 17. Flint, A. P. (1995) Reprod. Fertil. Dev. 7, 313–318. Servent, A., Boschiroli, M. L., Bourg, G., Fou- 10. Glaser, P., Frangeul, L., Buchrieser, C., Rusniok, 18. Bazer, F. W., Simmen, R. C. & Simmen, F. A. longne, V., Frutos, P., Kulakov, Y. & Ramuz, M. C., Amend, A., Baquero, F., Berche, P., Bloecker, (1991) Ann. N.Y. Acad. Sci. 622, 202–211. (1999) Mol. Microbiol. 33, 1210–1220. H., Brandt, P., Chakraborty, T., et al. (2001) 19. Schneider, H. (1991) Reprod. Fertil. Dev. 3, 345–353. 30. Hong, P. C., Tsolis, R. M. & Ficht, T. A. (2000) Science 294, 849–852. 20. Pouttu, R., Puustinen, T., Virkola, R., Hacker, J., Infect. Immun. 68, 4102–4107. 11. DelVecchio, V. G., Kapatral, V., Redkar, R. J., Klemm, P. & Korhonen, T. K. (1999) Mol. Micro- 31. Tauch, A., Schneiker, S., Selbitschka, W., Pu- Patra, G., Mujer, C., Los, T., Ivanova, N., biol. 31, 1747–1757. hler, A., van Overbeek, L. S., Smalla, K., Anderson, I., Bhattacharyya, A., Lykidis, A., et al. 21. Sola-Landa, A., Pizarro-Cerda, J., Grillo, M. J., Thomas, C. M., Bailey, M. J., Forney, L. J., (2002) Proc. Natl. Acad. Sci. USA 99, 443–448. Moreno, E., Moriyon, I., Blasco, J. M., Gorvel, Weightman, A., et al. (2002) Microbiology 148, J. P. & Lopez-Goni, I. (1998) Mol. Microbiol. 29, 12. da Silva, A. C., Ferro, J. A., Reinach, F. C., Farah, 1637–1653. 125–138. C. S., Furlan, L. R., Quaggio, R. B., Monteiro- 32. Schneiker, S., Keller, M., Droge, M., Lanka, E., 22. Cheng, H. P. & Walker, G. C. (1998) J. Bacteriol. Vitorello, C. B., Van Sluys, M. A., Almeida, N. F., Puhler, A. & Selbitschka, W. (2001) Nucleic Acids 180, 20–26. Alves, L. M., et al. (2002) Nature (London) 417, Res. 29, 5169–5181. 23. Charles, T. C. & Nester, E. W. (1993) J. Bacteriol. 459–463. 33. Perna, N. T., Plunkett, G., III, Burland, V., Mau, 175, 6614–6625. 13. Parkhill, J., Dougan, G., James, K. D., Thomson, 24. Puvanesarajah, V., Schell, F. M., Stacey, G., Doug- B., Glasner, J. D., Rose, D. J., Mayhew, G. F., N. R., Pickard, D., Wain, J., Churcher, C., Mun- las, C. J. & Nester, E. W. (1985) J. Bacteriol. 164, Evans, P. S., Gregor, J., Kirkpatrick, H. A., et al. gall, K. L., Bentley, S. D., Holden, M. T., et al. 102–106. (2001) Nature (London) 409, 529–533. (2001) Nature (London) 413, 848–852. 25. Geremia, R. A., Cavaignac, S., Zorreguieta, A., 34. Braun, W., Pomales-Lebron, A. & Stinebring, 14. Kalman, S., Mitchell, W., Marathe, R., Lammel, Toro, N., Olivares, J. & Ugalde, R. A. (1987) J. W. (1958) Proc. Soc. Exp. Biol. Med. 97, C., Fan, J., Hyman, R. W., Olinger, L., Grimwood, Bacteriol. 169, 880–884. 393–397. J., Davis, R. W. & Stephens, R. S. (1999) Nat. 26. Briones, G., Inon de Iannino, N., Roset, M., 35. Jones, S. M. & Winter, A. J. (1992) Infect. Immun. Genet. 21, 385–389. Vigliocco, A., Paulo, P. S. & Ugalde, R. A. (2001) 60, 3011–3014. 15. Tamas, I., Klasson, L., Canback, B., Naslund, Infect. Immun. 69, 4528–4535. 36. Caron, E., Liautard, J. P. & Kohler, S. (1994) J. A. K., Eriksson, A. S., Wernegreen, J. J., Sand- 27. LeVier, K., Phillips, R. W., Grippe, V. K., Roop, Leukocyte Biol. 56, 174–181. strom, J. P., Moran, N. A. & Andersson, S. G. R. M., II, & Walker, G. C. (2000) Science 287, 37. Harmon, B. G., Adams, L. G. & Frey, M. (1988) (2002) Science 296, 2376–2379. 2492–2493. Am. J. Vet. Res. 49, 1092–1097. COMMENTARY

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