BIOLOGICAL ASPECTS OF LYME DISEASE SPIROCHETES: UNIQUE BACTERIA OF THE BORRELIA BURGDORFERI SPECIES GROUP

BACKGROUND
Borrelia burgdorferi sensu lato is a group of at least twelve closely related species some of which are responsible for Lyme disease, the most frequent zoonosis in Europe and the USA. Many of the biological features of Borrelia are unique in prokaryotes and very interesting not only from the medical viewpoint but also from the view of molecular biology.


METHODS
Relevant recent articles were searched using PubMed and Google search tools.


RESULTS AND CONCLUSION
This is a review of the biological, genetic and physiological features of the spirochete species group, Borrelia burgdorferi sensu lato. In spite of a lot of recent articles focused on B. burgdorferi sensu lato, many features of Borrelia biology remain obscure. It is one of the main reasons for persisting problems with prevention, diagnosis and therapy of Lyme disease. The aim of the review is to summarize ongoing current knowledge into a lucid and comprehensible form.


INTRODUCTION
Lyme disease is a chronic multi-system infectious disorder and the most frequent infectious arthropod-borne disease found both in Europe and the United States.The number of yearly reported cases continually increases in a number of geographical areas (Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report, 2004).This may be due to the actual spread of the disease or alternatively, to improved diagnostic methods.Its signifi cance is shown by the frequency of publications in medical journals that focus on infectious conditions.From this point of view, Lyme's disease has assumed a position just behind acquired immunodefi ciency syndrome (AIDS) in recent decades 1 .
The disease is induced by spirochetes indicated totally as Borrelia burgdorferi sensu lato (Borrelia burgdorferi s. l.).This group of microorganisms comprises at least twelve species (Tab.1).Descriptions of new species and variants arises continously, so the number of species is probably not fi nal.
The disease has a vector character.Most of the time, it is transmitted to humans by infected ticks of the genus Ixodes, but Borrelia has also been found in the midgut of the mosquitoes, Culex pipiens 2,3 and Aedes vexans 4 .The presence of B. burgdorferi s. s. was earlier described in the midgut of some species of mosquitoes and other blood sucking fl ies 5 .Hence transmission may be possible by other arthropods than ticks alone.
In about 70 % of patients, the clinical picture of the disease starts as a pruriginous erythema at the site of the tick bite.Over time the erythema spreads out of the initial focus.The acute phase of the disease presents a picture of non-specifi c fl u-like symptoms.Untreated, the disease develops toward arthritis and/or dermal, neurological or cardiac complications.Involvement of the nervous system may manifest as diff use or focal neuropathy, painful radiculopathy, or less frequently, meningitis or encephalomyelitis.Neurological complications are frequently associated with infections caused by B. garinii and B. burgdorferi 6 .Among dermatological manifestations, typical are erythema, mentioned above, and acrodermatitis chronica atrophicans, which is caused mostly by B. afzelii 7,8 .Lymphocytoma and circumscribed scleroderma are less common.Ophthalmic disorders, most often intraocular infl ammations, aff ected by Borrelia have been described as well [9][10][11] .A number of theories on the possible association between Borrelia infection and other poorly-understood syndromes have been published: sarcoidosis 12,13 , chronic fatigue syndrome 14 and neurodegenerative disorders, e. g.Alzheimer's disease 15,16 .
Borreliosis is not only a problem in human medicine.Clinical forms have been found in a number of species of domestic animals, e.g.dogs, horses, cats and cattle.Interesting is the lack of clinical symptoms in forest animals.This may be the result of the long-term co-evolution of Borrelia and its natural hosts, such as deer, hares and various rodents [17][18][19] .

Basic microbiological characteristics
The genus Borrelia, Swellengrebel 1907 is a member of the family Spirochetaceae which contain three genera -Borrelia, Leptospira and Treponema.Three genospecies forming the species group B. burgdorferi s. s. commonly cause human disease.In the USA only B. burgdorferi s. s. cause the disease, in contrast to Europe where B. aff zeli and B. garinii are the most common.Some other species are irregularly pathogenic (Tab.1).These diff erences in Borrelia geographic distribution have not been explained to date.Some species of Borrelia, not belonging to B. burgdorferi s. l., are the cause of other human diseases, such as relapsing fever 33 .
Borreliae are unicellular spiral microorganisms without a rigid cellular wall 10-30 μm in length, 0,2-0,3 μm in diameter (Fig. 1).About 7-14 fl agella at each end of the bacterium are responsible for Borrelia mobility.They are localized in periplasmatic space (Fig. 2).The rarity is the B. sinica with only 2-4 fl agella, which may be the reason for its immobility 30 .The fl agella consist mainly of two types of fl agellin proteins -minor FlaA (38 kDa) and major FlaB (41 kDa) (ref. 34).The shape of Borrelia is formed by fl agellae that are twisted around the protoplasmatic cylinder but, in contrast to other bacteria, the Borrelia cell wall does not have an active role in shape determination.Targeted inactivation of the FlaB gene induces loss of motility and the "rod-like" form of these mutants 35 .Comprehensive reviews of the motility and cytoskeleton of spirochetes were published by Charon and Goldstein in 2002 (ref. 36) and Wolgemuth, Charon, Goldstein and Goldstein in 2006 (ref. 37).
The composition of the cell envelope is similar to gram negative bacteria with some expressive diff erences, e. g. absence of lipopolysaccharide 38 and an abundance of lipoproteins in the outer cell membrane 39,40 .As major lipid components of Borrelia membranes, identifi ed are two phospholipids -phosphatidylcholine and phosphatidylglycerol and two atypical glycolipids -1-O-palmi-toyl-2-O-oleyl-3-O-α-D-galactopyranosyl-sn-glycerol and Cholesteryl 6-O-palmitoyl-β-D-galactopyranoside.Both glycolipids are able to elicit the production of specifi c antibodies in mouse models.It was assumed, that these antibodies can cross-react with gangliosides and thus can be involved in the pathogenesis of neuroborreliosis.This aside, the synthesis of glycolipid based vaccines has been published [41][42][43] .
Cultivation of Borrelia requires unique formulation of growing media.Several modifi cations of Barbour-Stonner-Kely's medium (BSK) allow the cultivation of spirochetes in fl uid medium at high densities 44 .BSK is very complex medium containing mainly amino acids, cofactors, salts, N-acetyl-D-glucosamine and a number of variable components such as bovine serum albumin, neopeptone, yeast extract and rabbit serum.Suppliers and a batch of a large number of the above ingredients can strongly aff ect growth rate, as well as the antigenic and morphologic characteristics of Borreliae [45][46][47] .Standardized medium, named BSK-H, is available commercially.Unique is the ability of Borrelia to grow in an iron ion-depleted environment, although iron level is limiting for a number of pathogenic bacteria.This feature of Borrelia is caused by elimination of most iron dependent matalloproteins and substituting Mn for Fe in the remaining metalloproteins 48 .
For the cultivation of Borrelia on solid medium, the BSK solidifi ed by agarose is commonly used in a one or two layer modifi cation 49,50 .The cultivation of Borrelia on solid medium enables the separation of individual colonies and thus selection of various genetic variants.Since some Borrelia strains do not grow in colonies on solid media, the limiting dilution method was developed for selection of Borrelia during cloning (D. S. SAMUELS, pers.comm.).
Another typical feature, which is not clearly understood until now is the formation of non-helical forms of Borrelia.Various cystic forms, blebs and L-forms have been repeatedly described during cultivation in medium    with non-optimal pH, absence of serum, presence of antibiotics and in aged cultures, or generally under stress conditions.It seems that at least some of these transformations are reversible.A number of authors propose that these alternations are signifi cant in the pathogenesis, immune evasion and/or antibiotic resistance of at least B. burgdorferi s. s. 15,16,[51][52][53][54][55] .
Classic microbiological technologies are unsatisfactory for identifying Borrelia species and hence, molecular biology approaches (PCR, RFLP-PCR) prevail today.For taxonomy studies the DNA typing of OspA, OspC, 16S rRNA and 5S -23S rRNA spacer are frequently used approaches.

Genome structure
The spirochetes of the genus Borrelia are unique bacteria.One characteristic feature of this microorganism is its unusual genome which consists of one linear chromosome with an approximate size of one mega base and several circular and linear plasmids 39,[56][57][58][59][60][61][62][63] .The Borrelia chromosome is relatively small compared to other bacterial chromosomes whose size varies from 580 to 9 300 kbp 64 .In 1997 the genetic map of B. burgdorferi s. s.B31 was published 39 , and later completed by Casjens et al. 63 .
The B. burgdorferi s. s.B31 MI chromosome was investigated by Casjens et al. 63 and shown to contain 842 functional genes and only 1 pseudogene.Chromosomal genes code proteins necessary for replication, transcription, translation, energy metabolism, and transport of nutrients and ions.Neither chromosome nor plasmids contain genes for cellular biosynthetic reactions.This is the reason for the limited metabolic capacity of B. burgdorferi s. l., which lacks enzymes necessary for the biosynthesis of aminoacids, fatty acids, nucleotides and cofactors.This also explains the need for a complex medium containing all these components for the growth of the Lyme disease spirochetes.Of the other chromosomal genes, the most important are genes for DNA repair (mutS, mutL, uvrA, uvrB, uvrC and uvrD), genes for homologous recombination (recA, recBCD, sbsC, sbsD, recG, ruvAB, recJ), genes for heat shock response (groES, groEL, grpE, dnaJ, hslU, hslV, dnaK, htpG), and chemotactic factors-coding genes (cheR, cheW, cheA, cheY, cheB) (ref. 39).The ends of the chromosome are formed by covalently closed hairpin telomeres.This feature is unusual for prokaryotes, but it has been found on DNA of double stranded DNA viruses and on some eukaryotic mitochondrial DNA molecules 65 .
In addition to the chromosome, 21 extrachromosomal replicons have been described in B31 strain: 9 circular plasmids (cp) and 12 linear plasmids (lp) (Tab.2).This is the maximum number of plasmids described in bacteria to date 63 .Apart from this, two other plasmids (cp32-2 and cp 32-5) have been described in other subcultures of B31, which presumably means that the total number of plasmids in the original isolate B31 was 23 (ref. 66).In other strains of B. burgdorferi s. s., the number of cp32 plasmids varies between 6 (strain N40) and 10 (strain Sh-2-82) (ref. 67).Other unusual phenomena are the presence of a large number of pseudogenes on plasmids and low degree of identity between plasmid gene sequences Biological aspects of Lyme disease spirochetes: Unique bacteria of the Borrelia burgdorferi species group and sequences of genes known from other organisms.Linear chromosomes end in hairpin telomeres similar to the chromosome telomeres 68 .This form, unusual for prokaryotes, was originally described in N15 prophage of E. coli -linear plasmid molecule with the above features 69 .Apart from Borrelia, linear plasmids have been described in Streptomyces, Rhodococcus fascians, Nocardia opaca, and in Thiobacillius versutus up to now 70 .Covalently closed hairpin telomeres have been found on DNA of double stranded DNA viruses and on some eukaryotic mitochondrial DNA molecules 65 .
Interesting is the high degree of genetic redundancy between some plasmids.Cp32 plasmids share sequence homology with each other and with plasmids lp56, lp54 and cp9.It seems very likely that lp54 and lp56 arose from integration of cp32 into linear progenitors.On Borrelia binds tick gut epithelium via OspA-TROSPA interaction (1).Intake of human blood with plasminogen (2), which is subsequently bound on the OspA.Plasminogen after activation (3) helps Borrelia to pass through gut epithelium (4).Consequently Borrelia passes through haemocoel to salivary glands of tick (5).
During this time, Borrelia changes its outer proteins profi le and in the salivary glands binds immunosupresive Salp15 (6)."Armed" with this protein, Borrelia leaves tick (7) and enters to the human (8).  the other hand, cp9 is probably a fragment of ancestral cp32.Another truncated member of cp32 family, cp18 has been described in B. burgdorferi s. s. isolates N40 and 297(ref. 63,66,71,72 ).Other highly similar plasmids are lp5 and lp 21 (ref.63 ). Egers et al. 73,74 proved that cp32 plasmids contains prophage genome and they described bacteriophage mediated lateral gene transfer in B. burgdorferi s. s.This may be an important mechanism in Borrelia genome variability.
Glockner et al. 75 compared genome of B. burgdorferi s. s.B31 with genomes of B. garinii strain PBi (isolated from cerebrospinal fl uid) and B. afzelii strain BKo (skin isolate).Chromosomes of these three species have a high degree of similarity.Only three regions show greater differencesori replicator and genes for membrane proteins bmp and lmp.Apart from this, plasmid content varies widely between these species, whereas B. garinii and B. afzelii are more similar to each other than to B. burgdorferi s. s.Plasmids of these three species were divided into 6 orthologous groups.Higher similarity was found in cp26 analogs (B.garinii cp28, B. afzelii cp27) and in lp54 analogs (B.garinii lp59, B. afzelii lp60-1).Similarity in the other groups was signifi cantly lower.Some B. burgdorferi s. s. plasmids are not essential for in vitro Borrelia growth and thus can be easily lost when passaged out of its natural host.However, some of those genes are necessary during natural infection course and for Borrelia persistence in the host as demonstrated by Labandeira-Rey et al. 76,77 .They examined the infectivity of mutants lacking lp25 and lp28-1 in C3H mice and demonstrated that the loss of infectivity in lp25 -and elimination of lp28-1 -mutants from tissues coincident with the onset of specifi c IgM immune response.In immune-deficient SCID mice, the lp28 -mutants normally persist.This observation reveals the importance of some genes located on these plasmids: the gene for nicotinamidase (protein ) substantial for bacteria metabolism, located on plasmid 25 and the gene for VlsE (Vmp-like sequence, expressed) important for immune evasion [76][77][78][79][80] located on plasmid 28-1.Abberation in vls was associated with loss of infectivity of B. garinii 75 .Purser and Norris 81 tested 19 clonal isolates of B. burgdorferi s. s. of known plasmid pattern.They divided plasmids into three groups: the fi rst group -plasmids required for infectivity (lp25, lp28-1), the second group -plasmids which were missing randomly in infective or non-infective isolates, thus not required for infectivity (cp9, cp32-3, lp21, lp28-2, lp28-4, lp56), and a third group -plasmids presented in all 19 isolates (cp26, cp32-1, cp32-7, cp32-4, cp32-6, cp32-8, cp32-9, lp17, lp28-3, lp36, lp38 and lp54).A few years later, clinical B. Burgdorferi s. s. isolates lacking lp38, lp56, cp32-1, cp32-8, and segment of 28-1 were described 82 .In contrast to the majority of bacteria, some Borrelia plasmids carry genes essential for survival, as was demonstrated for plasmid cp26 (ref. 83,84  coding at least three necessary proteins -GMP synthase (guaA), IMP dehydrogenase (guaB) and telomere resolvase (resT), the enzyme responsible for capping linear replicons with hairpin telomeres 85 .This is another speciality of the Borrelia genome as plasmids often carry genes that confer a selective advantage, but not genes necessary for the life cycle.Moreover, cp26 plasmid carries a gene coding for outer surface protein C (OspC), which is necessary for early stage of vertebrate host infection but is not required during growth in the tick vector or during in vitro growth [86][87][88] .Strother et al. 89 tried to identify plasmids of B. burgdorferi required for tick infection.They demonstrated that plasmids lp5, lp28-1, and cp9 are not required for tick infection, whereas loss of lp25 and lp28-4 is associated with reduced tick infectivity.Moreover it was discovered that absence of lp28-1 reduces transmission of Borrelia to tick salivary glands, the organ from which spirocheta is released to vertebrate host.
These data show that plasmids of B. burgdorferi s. l., especially linear ones, have some characteristics exceptional for common defi nition of "plasmid" -the redundant, mainly circular DNA molecule, conferring antibiotic resistance or other selective advantage and present in cells in variable number.In addition, linear plasmids have similar features to linear chromosome which supports the idea that at least some plasmids of B. burgdorferi s. l. should be regarded as "minichromosomes" 90 .

Outer membrane proteins and their signifi cance in the pathogenesis and transmission of the disease
Members of B. burgdorferi species group have a complex life cycle.All these species circulate between arthropod vector and vertebrate host.As mentioned above, the most frequent vectors are ticks of the genus Ixodes and vertebrate hosts originate from a broad range of avian and mammalian species.
During its life cycle Borrelia must adapt to two very diff erent environments -tick and vertebrate organisms.
Borrelia must be able to adhere and survive in the tick gut, pass by gut epithelium to the haemolymph and transport through salivary glands to the host bloodstream, must avoid the immune reaction and disseminate to the target tissue.During Borrelia transition between Ixodes and vertebrate, a key role is played by several outer surface proteins.Their expression is regulated in separate phases of Borrelia infection [91][92][93][94] .

Outer surface protein A
The most known is antagonism between expression of outer surface protein A (OspA) and OspC in B. burgdorferi s. s.The OspA was the fi rst described member of outer surface proteins -more precisely lipoproteins group 95 .OspA has molecular weight of 31 kDa.The gene for this protein is part of the bicistronic operon on plasmid lp54 expressed together with the gene for another outer surface protein -OspB, with a molecular weight 34 kDa 96 .These proteins are expressed in the midgut of tick, or in cultured Borrelia and are downregulated during transition to the vertebrate host 97 .The main function of OspA consists in binding to glycoprotein TROSPA present in tick gut epithelium (tick receptor for OspA).The biological function of TROSPA glycoprotein in tick is unknown.Using quantitative RT-PCR, expression of TROSPA was shown to be signifi cantly increased in Borrelia infected ticks.Another purported function of Borrelia OspA is the binding of plasminogen from the host blood during tick feeding.The phenomenon of plasminogen binding is described as the virulency factor in some other bacteria, e.g.Streptococcus, Salmonella, Haemophilus and Yersinia 99 .The presumed sense of OspA-induced plasminogen activation toward active plasmin is the misuse of its proteolytic activity for Borrelia invasion into the host tissues.Plasmin proteolytic activity is presumed to be important during Borrelia transit through tick gut epithelium to the haemocoel 100 .Consequently Borrelia pass through haemocoel to tick salivary glands from where spirochetes are exported to the bloodstream of vertebrate host.Garcia et al. 101 demonstrated that both OspA and OspB directly bind to vertebrate host complement receptor 3 (CD11b/ CD18) and so induce activation of immune system cells.OspA also activate immune system via binding to Toll-like receptors 1 and 2 (reviwed in Singh and Girschick 102 ).This may be one of the reasons for the downregulated of OspA/B expression during transmission to vertebrate host which minimizes the immune infl ammatory reaction in the early stage of disease.On the other hand, Crowley and Huber 103 discovered that expression of OspA in vertebrate host is stimulated in an infl ammatory environment, particularly after zymosan-induced infl ammation.Because the expression of B. burgdorferi s. s.OspA is noted in later stages of mammalian host infection, usually accompanied by arthritis 104 .It is not clear whether increased OspA expression during chronic infection is just the Borrelia response to otherwise induced host infl ammatory response to Borrelia or if Borrelia OspA expression is necessary for chronic infection.One explanation for active Borrelia OspA expression during chronic infection is the potential protective function of OspA for Borrelia cells.Bunkis & Barbour 105 cited OspA as the protease and acid resistant structure on Borrelia surface which shields Borrelia against proteases and low pH in such "unfriendly" places as the tick gut or infl amed mammalian tissue.
In contrast to an infl ammatory environment -antibodies downregulate OspA expression, irrespective of their specifi city 106,107 .Thus the regulation of OspA expression resembles positive feedback (infl ammation -protection) which must be regulated in Borrelia due to the stage of disease development by as yet not well-understood mechanisms.

Outer surface protein C
Another outer surface protein which is tightly linked to the vertebrate stadium of Borrelia infection is OspC.The OspC gene is located on cp26 plasmid and encodes a 23 kDa lipoprotein, previously known as pC 108 .Borreliae lacking this protein are not pathogenic but they can be cultured 88 .The function of this protein was unknown for a long time.Recently the ligand for OspC -tick salivary immunosuppressive protein Salp15 -was described 109 .It is a soluble component of tick saliva which suppresses the vertebrate host immune response against components of tick saliva during tick engorgement.Salp15 inhibits proliferation of CD4 + T cells by binding to extracellular domains of the CD4 molecule and inhibits calcium signals needed for activation of transcription factors NF-κB and NF-AT during T cell receptor mediated signaling in vivo as well as in vitro 110,111 .In this way Borrelia acquires an immunosuppressive coat of OspC-binded Salp15, which make them undetectable to the vertebrate immune system during the early stage of host colonization.Interestingly, the expression of Salp15 is elevated in infected ticks 109 as in the case of TROSPA mentioned above.The evolutionary reason for this tick cooperation with its own pathogen is open to question.Newly published data attribute to OspC plasminogen binding activity as was described for OspA above 112 .
However, OspC is not the only protein involved in immune evasion.An essential role is attributed to VlsE, the lipoprotein mentioned in the previous section.The gene for this protein consists of 15 silent vls cassetes, segments of which are recombined into the vlsE region during mammalian infection 113 .This results in antigenic variability of expressed lipoprotein and thus evasion of specifi c immune reaction.Recombination of silent fragments into vlsE region occurs only in vivo.Nevertheless signals involved in recombination are unknown.It was found that the number of amino acid changes is signifi cantly higher in immunocompetent C3H mice than in SCID mice and this suggests an infl uence of selective pressure 114,115 .

OspEF related proteins and CRASP
Borrelia is also armed against the activity of complement.And this arm again posses to the host of this bacteria.It has been demonstrated, that one B. burgdorferi s. s. outer surface lipoprotein -OspE, is responsible for Borrelia surface-binding of factor H in human blood.
Factor H is a human regulatory protein, inhibiting the activity of complement component C3b by means of facilitating its cleavage by factor I and competing with factor B required for formation of C3 convertase in an alternative pathway of complement activation.Factor H-like protein (FHL), the truncated form of factor H, has a similar function 116 .These mechanisms are used by mammalian cells to protect themselves against non-specifi c complement activation which could otherwise result in autoimmune damage.Thus Borrelia utilizes the host's own cell surface protective mechanisms.OspE is a member of so-called Erp's (OspEF related proteins), together with proteins coded by genes OspF, erpAB, erpCD, erpG, erpHY, erpIJ, erpK, erpLM, erpNO, erpPQ, and erpX.These genes are localized on cp32 plasmids.The exception is the erpX localized on plasmid lp56 (ref. 117,118 .The similarities of predicted products of these genes vary between 17-100 % and protein products have diverse ability to bind factor H in various animal species 119 .Apart from this observation, Kraiczy et al. 120,121 described a group of proteins capable of binding factor H and factor H -like protein (FHL) and designated them CRASPs (Complement regulatoracquiring surface protein).From analysis of diff erences between CRASP members, the authors explain the variable sensitivity of B. burgdorferi s. s., B. afzeli and B. garinii to the complement observed earlier by Breitner-Ruddock et al. 122 , who divided Lyme disease Borreliae into three groups: complement resistant (B.afzeli), complement sensitive (B.garinii) and complement partially sensitive (B.burgdorferi s. s.).CRASPs appear to be a heterogenic group, because later research showed that some CRASPs members belong to erp family 123 and at least one, CRASP-1 of B. burgdorferi s. s., does not 124 .

Adhesive proteins
Other B. burgdorferi s. s. group of outer surface proteins included in the pathogenesis of Lyme disease are adhesive molecules which bind vertebrate tissue structures.Decorin binding protein A and B (DbpA, DbpB) bind collagen-associated proteoglycan decorin.,Borrelia glycosaminoglycans binding protein (Bgp) binding glycosaminoglycans and Fibronectin binding protein (BBK32) binding extracellular protein fi bronectin and glycosaminoglycans 125,126 .The mediation of B. garinii adhesion to neuronal cell proteoglycans during neuroborreliosis has been recently attributed to OspA, the protein mentioned above 127 .Not fully undersood is role of hemagglutinating lectin B31LEC, but its function in host-pathogen interactions is supposed 128,129 .Borrelia adhesion is discussed in detail by Coburn et al. 130 and Grubhoff er et al. 131 .

Regulation of expression
The mechanisms of transcriptional regulation of these proteins are not fully understood but known is the essential role of alternative sigma factors -RpoN (σ 54 ) and RpoS (σ 38 , σ s ).RpoS expression is activated by RpoN cooperating with response regulatory protein (Rrp2) under conditions such as increased temperature or decreased pH.RpoS induces expression of temperature inducible Biological aspects of Lyme disease spirochetes: Unique bacteria of the Borrelia burgdorferi species group genes, like OspC, OspF and DbpA.In contrast, expression of OspA is dependent on housekeeping factor RpoD (σ 70 ) and is downregulated by the RpoS dependent pathway [132][133][134] .

CONCLUSIONS
In spite of considerable research progress on the biology of Borrelia in the most recent years, some features of this bacteria remain obscure.Further, most publications focus only on one species -Borrelia burgdorferi s. s.This may be due to the concentration of Borrelia research groups in the USA where Borrelia burgdorferi s. s. is the only pathogenic species of this group while the situation in Europe is diff erent.Continued research in this fi eld is important not only from the medical point of view but also because it yields valuable outcomes for understanding a number of general biological questions such as plasmid plasticity, antigenic variation and vector-pathogen-host interactions.

Fig. 3 .
Fig. 3. Transmission of Borrelia burgdorferi from tick to human.Borrelia binds tick gut epithelium via OspA-TROSPA interaction (1).Intake of human blood with plasminogen (2), which is subsequently bound on the OspA.Plasminogen after activation (3) helps Borrelia to pass through gut epithelium (4).Consequently Borrelia passes through haemocoel to salivary glands of tick(5).During this time, Borrelia changes its outer proteins profi le and in the salivary glands binds immunosupresive Salp15(6)."Armed" with this protein, Borrelia leaves tick(7) and enters to the human(8).Table in the upper right corner illustrates human immune reaction to Borrelia.Gray arrows indicate increasing or decreasing of immune response.Abbreviations are interpreted in the text.

Fig. 5 .
Fig. 5. Adhesion of Borrelia to tick and human tissues.Symbols are explained in Fig. 4.

Fig. 4 .
Fig. 4. Illustration of changing expression of OspA and OspC during Lyme disease transmission.Expression of Osp´s in the natural Borrelia´s hosts is poorly known.Borrelia express OspA during tick infection, where OspA acts as a tick specifi c adhesive molecule.During transmission to human, expression of OspA decreases and expression of OspC increases.Some reports proved expression of OspA during later stages of human infection.

Table 1 .
Biological aspects of Lyme disease spirochetes: Unique bacteria of the Borrelia burgdorferi species group Species of B. burgdorferi complex.

Table 2 .
39,63cons of B. burgdorferi s. s. strain B 31 MI39,63 a -Percent of plasmid occupied by genes and pseudogenes Table in the upper right corner illustrates human immune reaction to Borrelia.Gray arrows indicate increasing or decreasing of immune response.Abbreviations are interpreted in the text.