ISSN 0891-4168, Molecular Genetics, Microbiology and Virology, 2009, Vol. 24, No. 4, pp. 169–176. © Allerton Press, Inc., 2009. Original Russian Text © V.V. Kutyrev, G.A. Eroshenko, N.V. Popov, N.A. Vidyaeva, N.P. Konnov, 2009, published in Molekulyarnaya Genetika, Mikrobiologiya i Virusologiya, 2009, No. 4, pp. 6–12.

REVIEWS

Molecular Mechanisms of Interactions of Plague Causative Agents with Invertebrates V. V. Kutyrev, G. A. Eroshenko, N. V. Popov, N. A. Vidyaeva, and N. P. Konnov Microbe Russian Research Institute, Universitetskaya ul. 46, Saratov, 410005 Russia e-mail: [email protected] Received June 16, 2009

Abstract—The review compiles literature data along with the results of original experimental studies of the molecular bases of interactions of plague causative agent with invertebrates. Data on the life cycle of plague causative agent, the organization of its genome, and molecular genetic mechanisms of its survival in carrying fleas and on the cuticles of nematodes are presented. The experimental data on the capacity of biofilm formation on both biotic and abiotic surfaces of basic and nonbasic subspecies of Yersinia pestis are presented. The mechanisms of horizontal and vertical transmission of plague causative agent are also discussed. Some proposals are made in connection with a new participant, i.e., parasitic and carrier nematode, in complex parasitic biocoenosis. Key words: plague causative agent, invertebrate animals, molecular genetic mechanisms of interaction DOI: 10.3103/S0891416809040028

INTRODUCTION The plague represents a strongly dangerous infective disease with natural hotbeds and transmissive mechanism of transfer of causative agent. In total, 233 mammal species serve as natural bearers of plague, while 162 species and 18 subspecies of fleas can be carriers [6, 12]. This disease still represents true danger, since multiple natural hotbeds of plague exist, 42 of which are situated in the Russian Federation and neighboring countries [12]. According to WHO data, more than 2000 cases of human plague are registered annually worldwide [35]. Life cycle of Y. pestis. As classical concepts state, the transfer of plague causative agent occurs according to the rodent–flea–rodent pattern. After a flea bites a sick animal, cells of the plague microbe enter its intestine with the blood and multiply, accumulate in the insect’s proventriculus, and are transferred to the healthy rodent with the bite of the infected flea, leading to the development of the disease in the animal. Other fleas are infected by feeding on this animal and transfer the infection to new hosts, thereby further distributing the infection. After the flea’s bite of an endothermal animal or human, Y. pestis cells reach the nearest regional lymph node and multiply in it, causing the manifestation of classical symptoms of the disease, i.e., acute lymphadenopathy and the formation of buboes (Fig. 1) [19, 38]. The bubonic plague develops, which, in the case of the absence of treatment, progresses into septicemia leading to the systemic distribution of the causative agent and death caused by sepsis. If Y. pestis reaches the lungs, lung plague develops, which is the most conta-

gious form of the disease, since it can distributed through droplets. Data were recently obtained that indicate that the plague microbe can interact, not only with the carrying insect, but also with other invertebrate animals. For example, the capacity of Y. pestis to multiply on nematode (representatives of other invertebrate class) cuti-

169

Rodent infected with Y. pestis Y. pestis

Infected flea Y. pestis

Bite Lymph nodes Y. pestis

Bubonic plague

Blood Y. pestis

Septicemic plague Lung Lung plague

Fig. 1. Stages of Y. pestis transmission from infected flea to human.

170

KUTYREV et al.

cles was demonstrated [21, 28]. It has been proposed that nematodes and possibly other members of soil biocoenosis can participate in the conservation of the plague microbe in the outer environment [9, 13]. Organization and evolution of Y. pestis genome. A comparative analysis of the genome of plague causative agent evolved relatively recently from the related enteropathogenic bacterium Yersinia pseudotuberculosis, the pseudotuberculosis causative agent, which is widely distributed, e.g., in soil, and transferred alimentarily with infected food and water and able to cause gastroenteritis and mesenterial lymphoadenitis in humans [16, 20, 32, 38]. The genome of plague causative agent is composed of the chromosome of 4.65 Mb and three nonconjugative plasmids, via pCad (70.3 kb), pFra (96.2 kb), and pPst (9.6 kb) [32]. One of these plasmids, the plasmid of calcium dependence pCad (synonyms pYV, pCD), is common between Y. pseudotuberculosis and other enteropathogenic microbe Yersinia enterocolitica. The presence of this plasmid is obligatory for the virulence of Y. pestis. Plasmid genes encode the synthesis of the Yersinia outer proteins (Yops) effector proteins and the type-III secretion system, which participate together in the suppression of phagocyte activity in cells of mammalian immune systems [34]. Two other plasmids, pFra (synonym pMT1) and pPst (synonym pPla or pPCP1), are specific [10, 34]; pFra plasmid encodes the synthesis of antigen F1 capsule and mouse toxin Ymt, while pPst determines the synthesis of bacteriocin pesticin and plasminogen activator, which exhibits two activities at different temperatures, i.e., fibrinolytic (37°C) and plasmocoagulase (28°C). The obtaining of pFra and pPst plasmids via the lateral transfer of genetic information from unknown donors appeared to be key stage in the rapid evolution of enteropathogenic bacterium with fecal-oral distribution into systemic pathogen distributed with flies [16, 19, 23, 32]. A comparison of the chromosomes of Y. pestis and Y. pseudotuberculosis revealed no new virulence genes that determine acute infection caused by plague causative agent in its genome. Conversely, it was found that this microbe is deficient in some functions that are obligatory for effective intermediate metabolism [19, 20, 32]. The genome of Y. pestis contains multiple pseudogenes that represent rudiments of genes that were functionally active in the pseudotuberculosis causative agent. The structure of these genes is disrupted with incorporations of multiple copies of IS elements, deletions, and insertions [7, 16, 20, 32]. This means that the acute form of the disease caused by Y. pestis may be connected with the loss of functions that are of no longer significant under new conditions of the existence of pathogenic Yersinia [17, 19]. Y. pestis is characterized by notable genetic homogeneity connected with the relatively recent origin of this species and its evolutionary youth. However, three biovars are described within the Y. pestis species,

including ancient (antiqua), medieval (medievalis), and eastern (orientalis). Each of these biovars has been the cause of three plague pandemics, which, combined, led to the death of more than 200 million people [16, 34]. Recently, D. Zhou et al. [39] proposed to describe one more biovar, microtus, whose strains survive on voles in two natural hotbeds in China and represent the results of a reduction in the evolution of strains of the medievalis biovar. However, original data indicate that, in some characteristics, microtus strains resemble nonbasic subspecies of Y. pestis that circulate in the territory of the Russian Federation and neighboring countries. In connection with this, further investigations are needed to clarify the systematic position of microtus strains. According to the domestic scheme of intraspecific classification [11], strains of plague causative agent are treated as five subspecies, including one basic and four nonbasic subspecies, i.e., Caucasian, Altai, Gissar, and Ulegei. Most strains of the basic subspecies are characterized by high virulence and epidemiological potential, while those of the nonbasic subspecies possess differentiated virulence and low epidemiological potential. Nonbasic subspecies may represent the most ancient forms of plague microbe and serve as intermediate forms between the pseudotuberculosis microbe and highly virulent basic Y. pestis subspecies; this proposal is partly confirmed by the absence of pPst plasmid in strains of the Caucasian subspecies [8]. The existence of a natural triad of bacterium, precursor (pseudotuberculosis causative agent), ancient forms of plague microbe (nonbasic Y. pestis subspecies), and highly virulent basic subspecies enables researchers to trace the molecular level of stages of evolutionary transformations of bacterial genome leading to the formation of a pathogen with a new means of transmission and novel mechanism of the development of the infection process. The key stage in this way is to obtain the capacity of Y. pestis to survive and multiply in an invertebrate organism, thus leading to the adoption of this bacterium as a causative agent of disease with a natural hotbed and transmissive character of distribution. Revealing the precise molecular mechanisms of interaction of plague causative agent with invertebrates would enable one to improve existing model concerning ways of conserving Y. pestis in the outer environment and the classic model of the epizootic plague process. Molecular Mechanisms of Interactions of Y. pestis with Carrier Insects The transfer of plague with blocked fleas has long been considered to be the only mechanism of Y. pestis transmission. In connection with this, molecular aspects of interactions between plague causative agent and insects are studied most precisely. After penetrating into the intestine of a flea with infected blood, Y. pestis multiplies and forms a bacterial aggregate, i.e., a plague block, which shuts the proven-

MOLECULAR GENETICS, MICROBIOLOGY AND VIROLOGY

Vol. 24

No. 4

2009

MOLECULAR MECHANISMS OF INTERACTIONS

triculus of the flea, thus making feeding difficult. Subsequent bites by the insect lead to fragments of bacterial aggregate being washed with newly obtained blood into the wound of the mammal; this effectively leads to the injection of bacteria under skin. Unlike plague causative agent, the pseudotuberculosis microbe lacks the ability to survive in a flea’s body for a long time [23]. Special experiments revealed that genes of pFra plasmid play a crucial role in the survival of Y. pestis in fleas. The ymt gene localized on this plasmid encodes Ymt protein and mouse toxin (lethal for mice) represents phospholipase D [29]. This Ymt protein has a size of 61 kDa, possesses phospholipase activity, and serves as a cytoplasm enzyme. Strains that lack plasmid pFra lose the ability to colonize the intestine of the flea [26]. A few hours after penetrating the flea intestines, Ymt– mutants of Y. pestis become rounded and disappear from the middle stomach of the insect. However, details of the mechanism of the action of the toxin on the carrier’s body remain obscure. The intercellular phospholipase D possibly inactivates the cytotoxic product of blood degradation or modifies its bacterial target [26]. It was previously proposed that the leading role in the formation of a bacterial (plague) block in a flea intestines is connected with the activator of plasminogen Pla encoded by pPst plasmid and that the plasmocoagulase activity of Pla results in fibrin precipitation and the formation of a blocking clot of blood in the proventriculus of flea [31]. However, it was later estimated that the bacterial block includes no fibrin and cannot be degraded with proteases or fibrinolitic enzyme plasmin [23]. Moreover, it was demonstrated that Y. pestis strains lacking pPst are capable of blocking fleas to the same degree as initial strains that bear this plasmid [24]. The role of surface protease Pla in the transmission of plague causative agent via a flea’s bite may be connected with its assistance in the dissemination of the microbe in the wound [23, 24, 36]. It was also revealed that bubonic, not septicemic, plague depends on Pla [36]; besides this, the presence of Pla enables plague causative agent to rapidly multiply in respiratory tracts and is obligatory for the development of lung plague. The key role in achieving the phenomenon of block formation is played by products of chromosome region of pigmentation (pgm region or hms region (from “hemin stroage locus”)). When cultivated at 28°C on medium containing hemin or Congo red pigment, plague causative agent forms pigmented colonies (Pgm+); their pigmentation is connected with the adsorption of these substances on the outer membrane [25, 34]. The formation of a block in a flea intestines correlates to the phenotype of pigment sorption. When infecting the flea Xenopsylla cheopis with mixed populations of Pgm+ (Hms+) and Pgm– (Hms–) cells of Y. pestis, it was observed that cells with Pgm+ phenotype form dense conglomerates that attach among acanthi (cuticle excrescences) of the flea’s proventriculus. Cells with the Pgm– phenotype multiply in the intes-

171

tine, since the loosely packed mass is easily washed away from acanthi with blood during subsequent feedings of insect [2]. Genes responsible for pigmentation characteristics are localized at the pgm site of the Y. pestis chromosome in the locus designated as hms. Besides the hms locus, the pgm region (102 kb) includes a motif of high pathogenicity and Yersinia HPI with a ybt region that encodes the synthesis of a syderophore-dependent system of iron utilization. The presence of a motif of pathogenicity with a ybt region, together with pCad plasmid, is necessary for the manifestation of the virulence of Y. pestis strains. Spontaneous nonpigmented strains that lack either an hms locus or a whole site of pigmentation were characterized by the complete absence of the ability to block the intestine of an X. cheopis flea [25], while rare mutants that lack the hms locus but possess the ybt region and sensitivity to pesticine (gene psn) were completely virulent for endothermous animals [30]. Role of biofilm in blocking fleas with Y. pestis cells. As was recently estimated, most bacterial species in natural conditions do not exist in a state of individual free cells; rather, they exist in the form of so-called biofilms, i.e., cell units surrounded by extracellular matrices that include exopolysaccharide [5, 14, 22]. Biofilms defend bacterial cells against different factors in the outer environment. It was recently demonstrated that blockages formed by plague causative agent in flea intestines represent bacterial biofilm [9, 21, 27]. Aggregates of cells of Y. pestis that block the intestines of carrying insects are surrounded by extracellular matrices that are positive for polysaccharide in histological tests. This fact confirms its nature as exopolysaccharide and, as a whole, corresponds to the definition of biofilm [18, 21, 27], meaning that plague blockages are composed of massive bacterial biofilm formed by Y. pestis on the acanthi of flea intestines, as well as that the blockage of a carrier insect by plague causative agent depends on the efficiency of bacterial-film formation in vivo (Fig. 2). At different stages of block formation, singular groups of plague microbe cells surrounded by biofilm matrices appear in middle stomach in a form of so-called particles. Tiny particles of plague microbe are excreted from the flea’s organism via defecation, while larger particles accumulate in the middle stomach, thus providing the constant excretion of fragments of Y. pestis biofilm in the outer environment by an infected flea. The capacity to form particles was found in many flea species, including those that do not exhibit block formation in experimental conditions [1, 3]. One cannot exclude the idea that this process, which is based on the plague microbes’s ability to form a biofilm on flea acanthi with the subsequent excretion of its fragments in the outer environment, may the underlie plague enzootic process [13] and provide long-term storage of plague microbe in soil, including in flea excrement and corpses [1, 3].

MOLECULAR GENETICS, MICROBIOLOGY AND VIROLOGY

Vol. 24

No. 4

2009

172

KUTYREV et al.

tion, the plague microbe has evolved the capacity to form biofilms on different surfaces, which it inherited from the pseudotuberculosis causative agent, and adapted it into ability to produce blockages in the proventriculus of carrier insect. This unique mechanism of biofilm formation in flea intestines cannot be found in other pathogenic bacteria transferred with carrying insects. The temperature-dependent (up to 26°C) regulation of biofilm formation plays an important role in the causative agent’s biology, since the expression of this trait is obligatory for existence in both invertebrates and the outer environment, not in mammals (37°C) in which other mechanisms of survival and protection of Y. pestis cells are active [9, 19, 24, 36, 38].

BF

BF

AC BF AC

Fig. 2. Scanning microphotograph of intestine of flea X. cheopis with massive biofilm of Y. pestis on acanthi (AC) of intestine. BF, biofilm of plague microbe. Obj. ×1300.

Genes of the hms locus of the Y. pestis chromosome are needed for the formation of both biofilms and blockages in flea intestines [18, 21, 27]. Despite the absence of direct arguments that genes hms encode proteins involved in the synthesis of basic component of biofilm matrix (exopolysaccharide), many facts indirectly confirm this proposal. A comparison of the amino-acid sequence of Hms proteins of Y. pestis with proteins of other bacteria is evidence of the similarity of hms genes to genes of glycosyl transferases and polysaccharide deacetylases, which encode bacterial proteins involved in the synthesis of extracellular polysaccharides [24, 27]. The hms locus includes four structural genes, hmsHFRS. Two other regulatory genes, hmsT and hmsP, are localized on Y. pestis chromosomes far beyond the boarders of locus hms [18]. It has been estimated that HmsH and HmsF represent proteins of outer membrane, while HmsR, HmsS, and HmsT contain transmembrane domains and are localized in inner membrane [34]. The product of regulatory gene hmsT contains the predicted transmembrane domain and the GGDEF domain [18], which is found in some bacterial proteins that regulate the synthesis of extracellular cellulose. The level of HmsT, HmsH, and HmsF production is significantly lower when cultivating Y. pestis at 37°C than at 26°C, which is most likely responsible for the expression of the temperature dependence of the Pgm (Hms) trait [34]. Thus, the presence of three genes of hms locus is obligatory for the manifestation of three characteristics of Y. pestis, via the formation of pigmented colonies on medium with pigment, the formation of biofilm, and the production of plague blockage in flea intestines. During its evolu-

In addition to hms genes, the plague microbe also needs numerous genes whose products may participate in biofilm formation. One of these genes is gmhA, which encodes the biosynthesis of the heptose enzyme (conservative component of core part of lipopolysaccharide), phosphoheptose isomerase [21], and the genes of polyamine putrescine biosynthesis. It was demonstrated that gmhA mutants of Y. pestis are barely able to block fleas; this phenomenon may be connected with the lower production of exopolysaccharide rather than the alteration of its content or structure [21]. The double deletion mutant ΔspeA ΔspeC (genes responsible for putrescine biosynthesis) has the same defect in biofilm formation as hms (including hmsT and hmsP) mutants [33]. Although the pseudotuberculosis microbe is able to produce biofilms on different surfaces, unlike the plague mocrobe, it cannot form biofilms in flea intestines [23]. The presence of a negative regulator of biofilm formation in fleas and the rcsA gene was recently estimated in the genomes of both causative agents; this gene is functionally active in Y. pseudotuberculosis, but represents the pseudogene in the plague microbe [37]. The absence of the activity of a negative regulator of biofilm formation in fleas enables the plague microbe to intensively multiply in the insect’s intestine. The transformation of rcsA in a pseudogene during the evolution of Y. pestis was most likely the result of negative selection than spontaneous changes in gene structure [37]. Because significant differences in formation of biofilm by plague and pseudotuberculosis microbes exist (as was previously demonstrated, due to different mechanisms of regulating this feature), we investigated the ability of Y. pestis strains of nonbasic subspecies to form biofilms in order to determine the evolutionary stages of the transformation of this process; these subspecies occupy intermediary positions between two causative agents. As a result of experiments, it was found that most studied strains of nonbasic subspecies (Caucasian, Altai, Gissar, and Ulegei [4]) form biofilms on abiotic surfaces (plastic dishes, Fig. 3); the presence of biofilms strongly correlates to the Pgm+ (Hms+) phenotype. In these biofilms, strains of nonbasic subspecies grow in the form of aggregates incorporated into

MOLECULAR GENETICS, MICROBIOLOGY AND VIROLOGY

Vol. 24

No. 4

2009

MOLECULAR MECHANISMS OF INTERACTIONS

1

2

3

4

5

6

173

7

8

9

Fig. 3. Biofilm formation on abiotic surface (plastic) by strains Y. pestis: 1, medium control; (2–4) Pgm– mutants of basic subspecies; (5–9) strains of Caucasian, Altai, Gissar, basic, and Ulegei subspecies, respectively.

(a)

(b)

(c)

Fig. 4. Study of biofilm formation by Y. pestis on C. elegans model. a, absence of biofilm formation in Pgm– mutants of basic subspecies; b, c, formation of biofilm (arrow) on head (b) and neck (c) parts of nematode C. elegans by strain of nonbasic (Gissar) subspecies. Obj. ×80.

the extracellular matrix, which correspond to the description of the biofilm [4]. We investigated the presence of certain genes that participate in biofilm formation in nonbasic subspecies via the genes of the hmsHFRS locus, regulatory hmsT and hmsP genes, gmhA genes (heptose biosynthesis), and speA and speC genes (polyamine putrescine biosynthesis); this study indicated that all listed genes are present in genomes of strains of nonbasic subspecies, probably in intact states [4]. The rscA gene (negative regulator of biofilm formation in fleas) in all studied Y. pestis strains of both basic and nonbasic subspecies, as compared with Y. pseudotuberculosis strains, has an insertion of 30 bp (294–323 bp from the start of the gene), which leads to a distortion in the gene structure and improves the rapid multiplication of plague microbe cells in the flea’s intestine. Thus, strains of nonbasic subspecies that represent early stages of Y. pestis evolution also possess the ability to form true biofilms. The deficiency of a negative regulator of biofilm formation in flea organism, the rscA gene, is already present in nonbasic subspecies of plague causative agent and evidence for the fact that a transition to a novel mode of transmission with the carrying insect led to chances (compared to the pseudotuberculosis microbe) in the regulation of this vital capacity; this alteration was due to the selective pressure on the fixation of constitutive biofilm formation in vivo.

Biofilm formation on cuticle of Caenorhabditis elegans. To study molecular mechanisms of biofilm formation by Y. pestis in vivo, the nematode model of C. elegans is now used [21, 28]. It was demonstrated that all Y. pestis strains and 20% of those of Y. pseudotuberculosis at low temperatures (up to 26°C) are capable of producing bacterial biofilm on the head and upper part of the body of C. elegans, thus disabling nematode nutrition [21, 28]. As for flea blocking, the presence of an hmsHFRS operon is necessary for biofilm formation on the cuticle of a nematode, which is evidence of the same content of biofilms in flea intestines and on the head of C. elegans. Staining of lectins confirms that the extracellular bacterial matrix on nematodes contains extracellular polysaccharide [28]. As biofilm formation may have significant differences between biotic and abiotic surfaces, we investigated biofilm formation by Y. pestis strains of both basic and nonbasic subspecies on C. elegans model. These studies demonstrated that biofilm formation on nematode’s cuticle strongly correlates with pigmentation phenotype. The Pgm– strains of basic subspecies produced no biofilm in these conditions (Fig. 4). The Pgm+ strains of basic and nonbasic subspecies are capable to construct biofilm on cuticle of nematodes C. elegans although expressivity of this trait was unequal in different strains [4]. Conclusively, most strains of non-

MOLECULAR GENETICS, MICROBIOLOGY AND VIROLOGY

Vol. 24

No. 4

2009

174

KUTYREV et al. Biofilms of Y. pestis (nematodes’ cuticle, particles of plague microbe) Imago of flea

Y. pestis

Humidity

26°C

Y. pestis

Larvae of nematodes Y. pestis

Rodent (bacteriemia) Y. pestis

Larvae of fleas Y. pestis

Blocked fleas Y. pestis

Pupa of flea Y. pestis

(a)

(b)

Infected rodents (plague epizooty)

Fig. 5. Mechanisms of horizontal (a) and vertical (b) transmission of Y. pestis in natural hotbeds of plague.

basic subspecies produce biofilm on abiotic (plastic) and biotic (nematodes’ cuticle) surfaces and contain genes that are (according to literature data) obligatory for its formation. Obviously, the possibility to produce the expressed biofilm in flea intestines and on the cuticle of nematodes was formed in Y. pestis at the early stages of species divergence (nonbasic subspecies), thus providing a new mechanism of transmitting the causative agent in invertebrates. Mechanisms of Origin and Development of Plague Epizooty The epizootic process in the case of plague is characterized by numerous peculiarities, some of which have not yet been precisely studied, including the explosive beginning of epizooties, which may arise simultaneously in different parts of plague hotbed, rapid distribution, and the periodic appearance of long-lasting periods between epizooties during which no infected animals can be found. Contemporary knowledge on the genetic specificity of plague causative agent and on molecular bases of its interactions with invertebrates present an opportunity to widen traditional views on the origin and development of epizooties caused by Y. pestis. The model of blocked fleas as a source of the distribution of causative agent according to the scheme of classical horizontal transmission, i.e., rodent–flea– rodent (Fig. 5a) can hardly explain the explosive discrete beginning of plague epizooties. Blocked fleas play an important role in the transfer of causative agents during epizooty development, not the beginning. The total accumulated data concerning the development of plague epizooties, together with previously

presented materials on molecular mechanisms of interactions between the causative agent and invertebrates, ensures that more complex and perfect natural processes exist, i.e., vertical transmission connected with the involvement of representatives of soil biota in the beginning of epizooties and with Y. pestis biofilms (plague microbe’s particles, cuticle of nematodes) (Fig. 5b). The precise mechanisms of vertical transmission are to be investigated, however one cannot deny obvious role of members of soil biocoenosis in initiation of plague epizooties. Probably reach of optimal combination of temperature and high soil humidity serves as signal for start of epizooties. We propose that the explosive beginning of epizooties are connected with the implementation of a novel mechanism of Y. pestis transfer in soil and holes according to the following series: biofilm of plague microbe–larva of free nematodes–larva of flea–pupa of flea–flea imago–rodent (Fig. 5b). The activity of this mechanism of vertical transmission leads to the primary infection of rodents followed by the distribution of infection in their population via infested fleas, which ensure the horizontal transmission of causative agent according to the scheme rodent–flea–rodent. Data on the ecology of nematodes (parasites of fleas) unambiguously confirm the reality of the emergence of causative agent from soil via the translarval transfer of plague microbe with nematode larvae to flea larvae, followed by the subsequent ending of their ontogeny and origin of flea imago infected by plague causative agent (Fig. 6). The observed explosive seasonal character of plague epizooties is due to simultaneous peaks of the number of larvae of free nematodes and fleas in soil. It is of special necessity that the opti-

MOLECULAR GENETICS, MICROBIOLOGY AND VIROLOGY

Vol. 24

No. 4

2009

MOLECULAR MECHANISMS OF INTERACTIONS

175

of existence in organisms of invertebrates and warmblooded animals. Throughout these processes, the evolutionary fixation of features that improve the activity of the epizootic process (intensive synthesis of biofilm in flea’s organism and on cuticle of nematodes) and virulence (selection of highly virulent strains Pgm+ variants with increase of level of bacteriemia in animals) occurred. ACKNOWLEDGMENTS The work was supported by grants of the Russian Foundation of Basic Research nos. 06-04-00100, 08-04-00731, and 08-04-12082 ofi. REFERENCES

Fig. 6. Presence of nematodes (double arrows) in intestine of flea X. cheopis infected with plague causative agent. Obj. ×1300 .

mum living conditions of larvae of fleas and nematodes, as well as conditions of biofilm formation by the plague microbe, are completely the same. Epizooties of plague most often aris in moist sites enriched with organic substances (declivities, bottoms of gullies, loci near wells), i.e., in biotopes optimal for soil nematodes. All of these facts together can explain the expressed seasonal character of plague epizooty development, together with dependence of the epizootic activity of natural hotbeds on climatic factors and evidence that nematodes represent an obligatory part in the ecology of plague causative agent. When protected with biofilm, plague causative agent can be stored in the soil of rodent holes (particles of plague microbes, nematode cuticles, and protozoa) for long times [1, 3, 13]. In natural plague hotbeds, one can sporadically register both individual infected animals (fleas mostly) and explosive epizooties that involve all background species of rodents and their ectoparasites. In our opinion, this mechanism underlies the observed polyhostal and polyvector character of natural plague hotbeds. The selection of Pgm+ Y. pestis strains occurs during the formation of plague-microbe biofilm and in warmblooded animals, with Y. pestis strains conserving the pigmentation locus with genes of the hms and ybt regions, which are necessary for the survival of the causative agent under different conditions. As a result, the evolution of plague microbe as the causative agent of a highly dangerous disease with natural hotbeds and its transmissive character was followed by the obtainment of new genetic information that ensured its survival in invertebrates, as well as by the adaptation of features obtained from an enteropathogenic precursor (pseudotuberculosis causative agent) to new conditions

1. Bazanova, L.P. and Innokent’eva, T.N., Med. Paraz. Paraz. Bolezni, 2008, no. 3, pp. 54–60. 2. Bibikova, V.A. and Klassovskii, V.N., Peredacha chumy blokhami (Plauge Transmission by Fleas), Moscow, 1974. 3. Velichko, L.N., Kondrashkina, K.I., Ermilov, A.P., et al., Probl. Osobo Opasn. Inf., 1978, vol. 6, no. 6 (64), pp. 51–53. 4. Vidyaeva, N.A., Eroshenko, G.A., Shavina, N.Yu., et al., Probl. Osobo Opasn. Inf., 2009, no. 1, pp. 32–34. 5. Il’ina, T.S., Romanova, Yu.M., and Gintsburg, A.L., Genetika, 2004, vol. 40, no. 11, pp. 1–12. 6. Karimova, T.Yu. and Neronov, V.M., Prirodnye ochagi Paleoarktiki (Natural Foci in Palearctic), Moscow, 2007. 7. Kukleva, L.M., Protsenko, O.A., and Kutyrev, V.V., Mol. Genet. Mikrobiol. Virusol., 2002, no. 1, pp. 3–7. 8. Kukleva, L.M., Eroshenko, G.A., Shavina, N.Yu., et al., Mol. Genet. Mikrobiol. Virusol., 2008, no. 2, pp. 32–36. 9. Kutyrev, V.V., Konov, N.P., and Volkov, Yu.P., Vozbuditel’ chumy – ul’trastruktura i lokalizatsiya v perenoschike (Plaque Pathogen: Ultrastructure and Localization in the Carrier), Moscow, 2007. 10. Kutyrev, V.V., Popov, Yu.A., and Protsenko, O.A., Mol. Genet. Mikrobiol. Virusol., 1986, no. 6, pp. 3–11. 11. Kutyrev, V.V. and Protsenko, O.A., Probl. Osobo Opasn. Inf., 1998, no. @, pp. 11–12. 12. Onishchenko, G.G., Kutyrev, V.V., Popov, N.V., et al., Prirodnye ochagi chumy Kavkaza, Prikaspiya, Srednei Azii i Sibiri (Natural Foci of Plaque in the Caucasus, Caspian Region, Middle Asia, and Siberia), Onishchenko, G.G. and Kutyrev, V.V., Eds., Moscow, 2004. 13. Popov, N.V., Sludskii, A.A., Udovikov, A.I., et al., Zhurn. Mikrobiol. Epidemiol. Immunol., 2008, no. 4, pp. 118–120. 14. Romanova, Yu.M., Alekseeva, N.V., and Smirnova, T.A., Zhurn. Mikrobiol. Epidemiol. Mikrobiol., 2006, no. 4, pp. 38–42. 15. Suleimenov, B.M., Mekhanizm enzootii chumy (Mechanism of Plaque Enzooty), Almaty, 2004. 16. Achtman, M., Morelli, G., Zhu, P., et al., Proc. Natl. Acad. Sci. USA, 2004, vol. 101, pp. 17837–17842. 17. Bearden, S.W., Sexton, C., Pare, J., et al., Microbiology, 2009, vol. 155, no. @, pp. 198–209.

MOLECULAR GENETICS, MICROBIOLOGY AND VIROLOGY

Vol. 24

No. 4

2009

176

KUTYREV et al.

18. Bobrov, A.G., Kirillina, O.A., Forman, S., et al., Environ. Microbiol., 2008, vol. 10, no. 6, pp. 1419–1432. 19. Brubaker, R.R., Microb. Ecol., 2004, vol. 47, pp. 293– 299. 20. Chain, P.S., Carniel, E., Larimer, F.W., et al., Proc. Natl. Acad. Sci. USA, 2004, vol. 191, no. 38, pp. 13826– 13831. 21. Darby, C., Ananth, S.L., Tan, L., and Hinnebusch, B.J., Infect. Immun., 2005, vol. 73, no. 11, pp. 7236–7242. 22. Hall-Stoodley, L., Costerton, J.W., and Stoodley, P., Nat. Rev. Microbiol., 2004, no. 2, pp. 95–108. 23. Hinnebusch, B.J., Curr. Issues Mol. Biol., 2005, vol. 7, no. 2, pp. 197–212.

24. Hinnebusch, B.J., Fisher, E.R., and Schwan, T.G., Infect. Immun., 1998, vol. 178, pp. 1406–1415. 25. Hinnebusch, B.J., Perry, R.D., and Schwan, T.G., Science, 1999, vol. 273, no. 5273, pp. 367–370. 26. Hinnebusch, B.J., Rudolf, A.E., Cherepanov, P., et al., Science, 2002, vol. 296, pp. 733–735. 27. Jarett, C.O., Deak, E., Isherwood, K.E., et al., J. Infect. D, 2004, vol. 190, pp. 783–792. 28. Joshua, G.W.P., Karlyshev, A.V., Smith, M.P., et al., Microbiology, 2003, vol. 149, no. @, pp. 3221–3229. 29. Kuturev, V.V., Vidyaeva, N.A., Bobrov, A.G., et al., Bacterial Protein Toxins, Jena–Stuttgart, 1997.

MOLECULAR GENETICS, MICROBIOLOGY AND VIROLOGY

Vol. 24

No. 4

2009