Plague Virulence

J. Med. Microbiol. Ð Vol. 50 .2001), 1015±1017 # 2001 The Pathological Society of Great Britain and Ireland ISSN 0022-2615

EDITORIAL

Plague virulence

Plague is usually regarded as a disease of antiquity, the change in lifestyle from an enteric pathogen to a Black Death having killed a third of the population of systemic pathogen transmitted by an insect vector. Y. Europe in the Middle Ages. However, as the outbreak pestis and Y. pseudotuberculosis both possess a large in Surat reminded the world, plague is still with us, plasmid, pCD1, encoding a type III protein secretion circulating in zoonotic reservoirs on most major system, but Y. pestis possesses two additional viru- continents. The advent of long-haul mass transport lence-associated plasmids ± pPst and pFra [6]. The coupled with the rise of multi-resistant strains of pPst plasmid carries the pla gene encoding a plas- Yersinia pestis, the causative agent of plague, means minogen activator, and pFra encodes the capsular F1 that local outbreaks pose a global threat. Thus plague is antigen and the murine toxin. The pFra plasmid shows an international public health risk and is a noti®able a high level of similarity to the recently sequenced disease. In 1997, for example, the World Health Org- cryptic plasmid pHCM2 from the enteric pathogen anization .WHO) recorded 4370 cases [1]. Salmonella enterica serovar Typhi [7]. Many potential pathogenicity islands were also identi®ed in the Y. Y. pestis is normally a pathogen of rodents. Although pestis genome [8], including one that carried a second usually transmitted by the bite of an infected ¯ea to type III secretion system. Many of the virulence genes produce bubonic plague, it is highly infectious by the possessed by Y. pestis appear to play a role in ¯ea aerosol route to produce pneumonic plague. Patients transmission instead of, or in addition to, a role in the respond well to antibiotic therapy provided that it is mammalian host. Interesting examples are the haemin administered at an early stage of infection, although storage .hms) locus and pla. The hms locus is required antibiotic multi-resistant strains are emerging [2]. Un- for ¯ea transmission, but not for virulence in the treated bubonic plague has a case fatality rate of up to mammalian host [9]. Y. pseudotuberculosis is HmsÀ, 50%, whereas pneumonic plague is almost always fatal although genes of the hms locus have been identi®ed in within 3 days. Pneumonic plague patients generate one Y. pseudotuberculosis strain [10], but phenotypi- large quantities of infectious aerosols, posing the risk cally this strain was hmsÀ due to a mutation in one of of epidemic spread. Antibiotic therapy must be started the genes. The pla gene product acts as a coagulase to immediately if exposure to a pneumonic plague patient produce blockage of the ¯ea at temperatures below is suspected, as once the symptoms of pneumonic 308C, but exhibits ®brinolytic activity at higher plague are apparent therapy is usually ineffective. A temperatures, possibly as a result of post-translational killed whole-cell vaccine is available, but the short- modi®cation [11]. Pla probably plays a role in vivo in lived and variable protection it affords combined with dissemination as Pla-defective mutants are attenuated the high level of adverse reactions make it unsuitable and are able to produce only a localised infection for general use. A second-generation vaccine based on following subcutaneous inoculation in the mouse model puri®ed protein antigens, which will protect against of infection [12]. An excellent review is available both bubonic and pneumonic plague, is in development covering the literature up to 1996 on known virulence [3]. The epidemiology and distribution of the organism, factors of Y. pestis [13]. and the recommended surveillance and control meas- ures employed are reviewed in detail in the WHO Whilst the organism has acquired additional virulence Plague Manual [1]. genes, the bacterium also has many pseudogenes [8] in pathways that are no longer essential after adaptation to Y. pestis is thought to have evolved from the en- its new niche. The organism exhibits a range of auxo- teropathogen Y. pseudotuberculosis [4, 5]. Although trophies [6, 14] and, therefore, requires rich media for almost identical genetically, the two organisms produce growth in the laboratory. Interestingly, although inter- markedly different diseases. Such changes in patho- ruption of the linear aromatic amino acid biosynthetic genicity in other bacteria have been associated with the pathway has been shown to be attenuating for a range acquisition of additional genetic material, such as of pathogenic bacteria, an aroA mutant of Y. pestis pathogenicity islands and plasmids, which broaden its retained virulence for mice [15]. There are three host range and improve its ability to overcome the recognised biovars of Y. pestis, separated by their host's defences or to cause tissue damage. In the case ability to ferment glycerol and to reduce nitrate, but of the evolution of Y. pestis, not only has the organism these differences are not re¯ected by alteration in gained virulence factors, but it has also undergone a virulence. Y. pestis appears to have passed an evo- 1016 EDITORIAL lutionary bottleneck in its transition from being an losis [21], Y. pestis possesses multiple N-acyl enteropathogen and, as has been seen previously for homoserine lactone-mediated quorum-sensing systems other pathogens such as Mycobacterium leprae and [22]. However, in Y. pseudotuberculosis quorum Rickettsia prowazekii, the organism is not maintaining sensing regulates expression of ¯agella, a phenotype genes that are no longer needed. An example of this that is irrelevant in the non-motile Y. pestis, as the phenomenon is the O antigen that is an essential ¯agellar and chemotaxis gene clusters are mutated [8]. virulence factor for many bacterial pathogens protect- ing the bacterial surface from the activity of comple- In summary, the plague organism has acquired genes to ment. Y. pestis does not produce an O antigen as the broaden its host range and to allow transmission by gene cluster is inactivated by multiple mutations ¯eas. Furthermore, many non-essential genes have [5, 16]. The basis for serum resistance of Y. pestis in become inactivated due to a lack of selective pressure the absence of an O antigen has not been de®ned, to maintain them. In its current niche, the pathogen although it may be due in part to surface proteases such exists in relative genetic isolation, unable to restore the as Pla cleaving complement components [12]. array of pseudogenes it now possesses, and thus is probably in an evolutionary dead-end. The divergence Because of the highly varied environments encountered from Y. pseudotuberculosis is thought to have occurred in the ¯ea/mammal lifestyle, the expression of viru- relatively recently, and the two species retain suf®cient lence and transmission factors must be carefully genetic similarity to elicit a proposal to reclassify them controlled. Temperature has been shown to be an as two related subspecies [23]. Despite the relatedness important signal in regulation of gene expression by Y. of the two species, the effect of mutations on the pestis. The transcription and secretion of Yops, the phenotype of the enteric pathogen cannot be assumed effectors of the pCD1 type III secretion operon, occurs to have a similar effect on Y. pestis because of the only at 378C. Also, the bacteria would be capsulate in markedly different pathogenesis of the two species. the mammalian host but not in the ¯ea, as a result of repression of the caf1 operon at lower temperatures PETRA OYSTON [13]. No environmental niche has been identi®ed Microbiology, Biomedical Sciences, outside the ¯ea and mammal, yet the genome sequence Dstl Chemical and Biological Sciences, Porton Down, Salisbury, Wiltshire SP4 0JQ reveals that the bacterium possesses many genes .e-mail: [email protected]) required for a cold-shock response. The high tempera- ture response protein, HtrA, has been identi®ed as a virulence factor in several organisms and htrAÀ References mutants of a number of species are attenuated [17]. Y. pestis possesses an htrA gene that appears to play a 1. Dennis DT, Gage KL, Grantz N, Poland JD, Tikhomirov E. central role in protein degradation at both 288C and Plague manual: epidemiology, distribution, surveillance and control. Geneva, World Health Organization. 1999. 378C. Inactivation of the gene did not result in a 2. Galimand M, Guiyoule A, Gerbaud G et al. Multidrug signi®cant degree of attenuation in the mouse model, resistance in Yersinia pestis mediated by a transferable plasmid. but the mutant was unable to grow at 398C [18]. N Engl J Med 1997; 337: 677±680. 3. Williamson ED, Eley SM, Stagg AJ, Green M, Russell P, Titball RW. A single dose sub-unit vaccine protects against pneumonic We have studied two regulatory systems which have plague. Vaccine 2000; 19: 566±571. been shown to play a role in virulence in other gram- 4. Achtman M, Zurth K, Morelli G, Torrea G, Guiyoule A, Carniel E. Proc Natl Acad Sci USA 1999; 96: 14043±14048. negative bacteria. The ®rst is the two-component 5. Skurnik M, Peippo A, ErvelaÈ E. Characterization of the O- response regulatory system, PhoPQ. In S. enterica antigen gene clusters of Yersinia pseudotuberculosis and the serovar Typhimurium, PhoPQ is essential for survival cryptic O-antigen gene cluster of Yersinia pestis shows that the plague bacillus is most closely related to and has evolved from within macrophages and for virulence in mice [19]. Y. pseudotuberculosis serotype O:1b. Mol Microbiol 2000; 37: Inactivation of the phoP gene of Y. pestis resulted in a 316±330. change in the expression of .20 proteins, the 6. Brubaker RR. Factors promoting acute and chronic diseases caused by Yersiniae. Clin Microbiol Rev 1991; 4: 309±324. regulation of which appeared to be affected by 7. Prentice MB, James KD, Parkhill J et al. Yersinia pestis pFra temperature, so that the Pho-regulated proteins were shows biovar-speci®c differences and recent common ancestry usually expressed at 288C but not at 378C [20]. This with a Salmonella enterica serovar Typhi plasmid. J Bacteriol 2000; 183: 2586±2594. suggests a role for PhoPQ in survival in the ¯ea or in 8. Parkhill J, Wren BW, Thomson NR et al. Genome sequence of transmission of the organism. In addition, the mutant Yersinia pestis, the causative agent of plague. Nature 2001 .in was more susceptible to killing by macrophages and press). 9. Hinnebusch BJ, Perry RD, Schwan TG. Role of the Yersinia less virulent in the mouse model, but the impact of the pestis hemin storage .hms) locus in the transmission of plague mutation was not as marked as had been observed for by ¯eas. Science 1996; 273: 367±370. Serovar Typhimurium probably re¯ecting the intracellu- 10. Jones HA, Lillard JW, Perry RD. HmsT, a protein essential for expression of the haemin storage .Hms‡) phenotype of Yersinia lar lifestyle of S. enterica versus the predominantly pestis. Microbiology 1999; 145: 2117±2128. extracellular lifestyle of Y. pestis. The second regula- 11. McDonough KA, Falkow S. A Yersinia pestis-speci®c DNA tory system we have studied is quorum sensing, a fragment encodes temperature-dependent coagulase and ®brino- lysis-associated phenotypes. Mol Microbiol 1989; 3: mechanism used by bacteria to monitor their popu- 767±775. lation density. Like its close relative Y. pseudotubercu- 12. Sodeinde OA, Subrahmanyam YVBK, Stark K, Quan T, Bao Y, EDITORIAL 1017

Goguen JD. A surface protease and the invasive character of 281±286. plague. Science 1992; 258: 1004±1007. 19. Groisman EA. The pleiotropic two-component regulatory system 13. Perry RD, Fetherston JD. Yersinia pestis ± etiologic agent of PhoP±PhoQ. J Bacteriol 2001; 183: 1835±1842. plague. Clin Microbiol Rev 1997; 10: 35±66. 20. Oyston PCF, Dorrell N, Williams K et al. The response 14. Burrows TW. Virulence of Pasteurella pestis and immunity to regulator PhoP is important for survival under conditions of plague. In: Henle W, Kikuth W, Meyer KF, Nauck EG, Tomcsik macrophage-induced stress and virulence in Yersinia pestis. J .eds) Ergebnisse der mikrobiologie immunitaÈtsforschung und Infect Immun 2000; 68: 3419±3425. experimentellen therapie. Band 37. Berlin, Springer-Verlag. 21. Atkinson S, Throup JP, Stewart GSAB, Williams P. A 1963: 59±113. hierarchical quorum-sensing system in Yersinia pseudotubercu- 15. Oyston PCF, Russell P, Williamson ED, Titball RW. An aroA losis is involved in the regulation of motility and clumping. Mol mutant of Yersinia pestis is attenuated in guinea-pigs, but Microbiol 1999; 33: 1267±1277. virulent in mice. Microbiology 1996; 142: 1847±1853. 22. Isherwood K, Atkinson S, Stewart G, Williams P, Titball R, 16. Prior JL, Parkhill J, Hitchen PG et al. The failure of different Oyston P. The identi®cation and characterization of a second strains of Yersinia pestis to produce lipopolysaccharide O-antigen quorum sensing locus in Yersinia pestis. In: Abstracts of the under different growth conditions is due to mutations in the O- IXth International Congress of Bacteriology and Applied antigen gene cluster. FEMS Microbiol Lett 2001; 197: 229±233. Microbiology. International Union of Microbiological Societies 17. Pallen MJ, Wren BW. The HtrA family of serine proteases. Mol 1999; 80: BP09.18. Microbiol 1997; 26: 209±221. 23. Bercovier H, Mollaret HH, Alonso JM et al. Intra- and 18. Williams K, Oyston PCF, Dorrell N, Li S-R, Titball RW, Wren interspecies relatedness of Yersinia pestis by DNA hybridization BW. Investigation into the role of the serine protease HtrA in and its relationship to Yersinia pseudotuberculosis. Curr Yersinia pestis pathogenesis. FEMS Microbiol Lett 2000; 186: Microbiol 1980; 4: 225±229.