ADAPTATION FACTORS OF BORRELIA FOR HOST AND VECTOR

Abstract: The life transmission cycle of B. burgdorferi requires migration of spirochetes from tick’s gut to its salivary glands during vertebrate’s blood sucking, penetrating to the vertebrate’s tissues and their colonization. A special feature of these bacteria, despite
its relatively small genome, is the ability to adapt in different host environments.

These unusual properties of borreliae are associated with large number of plasmids, which show
a high variability as a result of recombination with each other. Changes in the synthesis
of outer proteins are the first strategy of borreliae in avoiding the destructive effect of
the host’s immune system. Then, by colonizing tissues, they initiate production of Erp
and CRASP proteins, which bind regulators and components of complement and repress
the cytolytic effect of the host’s serum. Some evidences indicate that the spirochetes use
quorum sensing as a mechanism to control protein expression. B. burgdorferi probably
utilizes a LuxS/autoinducer-2-dependent quorum sensing mechanism. However, it is not
yet known how B. burgdorferi detect AI-2. Analysis of the results of expression studies of
the luxS gene shows that the molecular mechanisms of this phenomenon in B. burgdorferi
are only fragmentarily known. Continuation of quorum sensing studies may be essential
in improving the construction of vaccines as well as therapy of Lyme disease.
Address for correspondence: Department of Genetics, al. Piastów 40b, 71-065 Szczecin,
Poland. E-mail: boskot@univ.szczecin.pl
Key words: genome and adaptation of metabolism of Borrelia burgdorferi, quorum
sensing, Osp proteins, Erp and CRASP proteins, vls gene.
Received: 8 November 2007
Accepted: 18 January 2009
Ann Agric Environ Med 2009, 16, 1-8
2 Skotarczak B
The life transmission cycle of B. burgdorferi requires the
migration of spirochetes from the tick’s gut to its salivary
glands during vertebrate’s blood sucking, penetrating to
the vertebrate’s tissues and their colonization, achievement
of the chronic infection state, and finally, the colonization
of uninfected feeding ticks. This process is undoubtedly
connected with bacteria recognizing the environmental
signals, which modulate the expression of fundamental
genes that ensure success in adaptation. The unusual feature
of these bacteria is their ability to adapt and develop in
different host environments. Perhaps more unusual feature
of B. burgdorferi is that in spite of its relatively small genome,
it is able to carry out the course of events required
during transmission [26].
The special construction of the outer membrane, that is
the outer layer of the B. burgdorferi s.l. cell and is loosely
connected with lower lying structures, also expresses the
adaptation for carrying out such complicated cycle. Nontypical
of this membrane is fact that the genes encoding
its proteins (ospA and ospB) are in the extra-chromosomic
plasmid, and may be conducive for antigenic changes in
the structure of these proteins. What is more, this membrane
shows an ability to translocate from one end of the
cell to another. This is called capping or patching, and may
have significance in the phenomenon of adherence of the
microorganism to the host’s cells [6].
In the organism of a host-mammal, the bacteria from
Borrelia genus may use the host’s enzymes; such as plasminogen
and its activator, which can attach to the surface
of the bacteria’s cell [59]. The bacterium uses plasmine,
being the active form of plasminogen, for digesting the
extracellular glicoproteins with a large molecular mass.
Probably, the ability to digest the host’s tissue connections
enables the bacterium to translocate. Straubinger et al. [59]
suggest that B. burgdorferi actively migrates through the
tissues and fi nds a niche where it is able to survive, i.e.
period of antibiotic therapy, and it does not need mediation
of blood vessels for colonization of tissues. Therefore,
Borrelia avoids penetration of blood vessels because of
antibodies and the danger of phagocytosis by neutrophiles
and macrophages. Regarding this situation, the spirochetes
stay in the connective, poorly vascularized tissue. But such
a hypothesis of migration in tissues has not been confi rmed
by some reports in the literature about the detection of B.
burgdorferi in the humoral fluids in people, especially patients
with skin rash after exposure to ticks [53, 64].
Genome of Borrelia burgdorferi s.l.
Three pathogenic Borrelia species can permanently infect
humans and other mammals, despite the active immune
response of host. The ability to infect different animal species
probably results from the specifi c structure of these
bacteria’s genome. It is thought that this feature has a connection
mainly with factors encoded in enormous amounts
of plasmids found in borreliae’s genome [29], which show
a high variability as a result of recombination with each
other. In this bacterium, the chromosome is in the linear
form of the DNA molecule and is described as the fi rst
of this type in the world of bacteria. Its size is about 0.96
Mpz. There are two types of Borrelia’s plasmids: circular
(9) and linear (12), and they consist of about 613,000 bp.
The fi rst linear plasmids were detected 20 years ago, in
the fi rst place in yeast, then in bacteria including Borrelia.
Such a large number of plasmids have not been met in any
other bacterium [24]. What is more interesting, the unique
plasmids having less than 50% similarity to any other plasmids,
occur in all three pathogenic species Borrelia genus.
This phenomenon may be the cause of the adaptation of
these bacteria to different environments, as well as their
pathogenicity [29].
It has been found that the range of hosts species of vertebrates
and invertebrates is indirectly associated with factors
encoded mainly in huge number of plasmids found in
Borrelia. Another characteristic feature of these plasmids
is the abundance of parallel genes in them. During the passage,
the plasmids may be lost because of lack of the maintenance
pression. The incapability of Borrelia to survive in
the organism of the host may accompany such a loss [29].
To a certain degree, the chromosome is also engaged in
processes of fusion. It has been shown that the right end of
the chromosome in B. burgdorferi is variable because of
the ability to “catch” (attach) the plasmid material.
The presence of 853 genes encoding proteins that have
a meaning in the processes of replication, transcription,
translation, energetic metabolism and membrane transport,
were found in the chromosome of Borrelia spirochetes. On
plasmids, however there are mainly genes encoding outer
surface proteins, (Osp).
For example, the genome of B. burgdorferi s.s. species
consists of a linear chromosome including 910,725 bp, in
which G + C pairs constitute 28.6% and 21 plasmids (9
circular and 12 linear) consisting of about 613,000 bp. Today,
the genome of B. burgdorferi s.s. (B31) is completely
known, but in the case of B. garinii only the main chromosome
and some of the plasmids have been sequenced. The
comparative analysis of genomes have shown that three
genetic elements: chromosome and plasmids cp26 (circular)
and lp54 (linear) are common for both of these species
and are even strikingly collinear [29]. However, some
plasmids are limited to B. burgdorferi s.s. because neither
has been found in B. garinii, nor their traces in the form of
protein products.
Adaptation of metabolism of B. burgdorferi
to environmental conditions
Vector-borne bacterial pathogens, such as B. burgdorferi,
encounter different conditions as they are transmitted
to various hosts. Sensing changes in these environments
and accordingly, modulation of the metabolic level, is important
for adaptation and survival of B. burgdorferi within
Adaptation factors of Borrelia for host and vector 3
various hosts. The physiological fl exibility of spirochetes
also indicates the fact that they reproduce in many organs
and tissues of the infected mammals and birds. The occurrence
of these bacteria in different organs and tissues
of a warm blood host requires from them the synthesizing
of specifi c proteins appropriate for every kind of environment.
Among the proteins produced by B. burgdorferi
during infection, there are those which facilitate the
interactions between bacteria and selected cells of a host or
extracellular components. For the purpose of diverse synthesis
of proteins in the life cycle, the bacteria must have a
receptor system for detecting changes in the environment,
and a regulating one to regulate the expression of appropriate
genes and proteins. Such regulating mechanisms must
exist in order to control the expression of genes in the individual
bacterium, as well as in whole population.
Results of works on the genome of B. burgdorferi indicate
that these spirochetes have only rudimentary mechanisms
of own metabolism, and are almost completely dependent
on the host in the metabolism of fats, carbohydrates,
proteins, aminoacids and iron [27].
One of the factors controlling the ability of pathogenic
bacteria that colonize the host’s organism is the availability
of alimentary constituents, including ions of iron, which
take part in many essential biochemical processes. The
availability of these ions in structural fl uids and mucosal
membranes is limited and their concentration is too small
to maintain the vital functions of bacteria. Analysis of sequenced
genomes of pathogenic bacteria confi rmed their
great possibilities in range to receive ions of iron associated
with the activity of many copies of the same gene. However,
the strategy of B. burgdorferi differs from other bacteria,
namely, in the process of evolution, it eliminated the genes
encoding most of the enzymes dependent on the iron [43].
Many studies have shown that environmental factors
infl uence the synthesis of proteins associated with the infection
in mammals. They are produced in small amounts
at the temperature of 22°C, but when the amount is much
bigger – in temperatures of 22-34°C [54]. The spirochetes
are exposed to big changes of temperature while they are
living in the ticks. Ph is also an important parameter infl uencing
the synthesis of surface proteins in spirochetes.
Selective expression of membrane lipoproteins enables
survival in the host
Surface lipoproteins Osp (outer surface proteins) belong
to the most important protein antigens of B. burgdorferi
s.l., playing an essential role in the pathogenicity of Lyme
borreliosis, OspA, OspB, OspC, OspD, OspE and OspF
anchored in the outer membrane due to the occurrence
of the lipid group at the amino end [25]. The survival of
B. burgdorferi in tick and in the organism of mammals is
simplifi ed, at least, partly by the selective expression of
these lipoproteins. The fi rst strategy of borreliae, in order
to avoid the destructive activity of host’s immune system,
are changes in the synthesis of surface proteins (Osp)
and adaptation to different host’s microenvironments – in
mammal and in tick [11]. Many studies have demonstrated
that B. burgdorferi selectively shows an expression of individual
osp genes in the individual stages of its life cycle. In
this way, the expression of ospA and ospB is immediately
activated when the spirochetes penetrate into the vector,
arthropod. During the transmission from the vector to the
host-vertebrate, the expression of ospA and ospB is immediately
reduced and synthesis of OspC, DbpA and BBK32
is increased [21, 28].
Selective and temporal expression of ospA and ospB in
ticks suggests that these two proteins may be engaged in
the early colonization of spirochete and in the survival in
tick, vector. Studies by Pal et al. [47], showing that OspA
mediates in the adherence of spirochete to the tick’s gut
through binding TROSPA proteins, confi rm this theory and
indicate that the expression of the gene supports the maintenance
of the natural cycle of the spirochete. Furthermore,
the osp genes encoding proteins that have an antigenic
character occur in many allelic forms within each species
of B. burgdorferi s.l., which undoubtedly has a connection
with cheating the immune system of the host.
In the USA, the genes ospA and ospB are highly conservative
among isolates of B. burgdorferi [7]. They are
encoded by the DNA of linear plasmid pl54, activated by
the common promotor [10] and have a similar sequence
and structure [9]. Before the Lyme disease agent was detected,
OspA proteins had been an object of intensive studies,
whereas the role of OspB in the life cycle of borreliae
is not well known [41]. Earlier studies identifi ed a mutant
without ospB in the cloned population of B. burgdorferi
with one changed base in the sequence, and with deletion
of one nucleotide in the reading frame in the operon
of ospB gene [52]. These changes in the ospB gene cause
reduction of the expression and cutting of this protein in
such way that the ability of penetration and infection of
spirochete towards epithelium cells are reduced [52]. Other
studies suggest that OspB are present on the surface of B.
burgdorferi only in unfed ticks, and antigens against OspB
repress the colonization of borreliae in gut of I. scapularis
[25, 46]. Damage of the template in the operon ospAB repress
the colonization of B. burgdorferi and maintenance
in the tick’s gut [65]. Despite these studies clarifying the
important role of OspA in the in vivo spirochete-tick interaction,
the independent role of OspB in the life cycle of the
spirochete remains unclear.
The role of OspB protein in B. burgdorferi was investigated
by Neelakanta et al. [43] who obtained cells devoid
of this protein. They have shown that such borreliae are
able to infect and remain alive in mouse, and also migrate
to the feeding tick, adhere to and survive in its gut. But this
adherence is not tight, and the addition of one copy of ospB
gene to the wild strain of B. burgdorferi without OspB
strengthen it a great deal. The authors suggest that there
is proof that OspB plays an important role in I. scapularis,
4 Skotarczak B
and effective maintenance in this species of tick is dependent
on the expression of many genes of borreliae.
Erp and CRASP proteins help to avoid the immune
response of the host
It has not yet been found that these microorganisms, responsible
for the symptoms of the disease, produce toxins.
Through the colonization of tissues they initiate the occurrence
of the infl ammatory state, and at the same time, very
effectively avoid the destructive effects of the host’s immunological
mechanisms. One such defence mechanism is
the production of Erp and CRASP proteins, which through
binding the complement regulators – factor H and FHL-1,
as well as C3 C3b, C3c and C3d components, repress the
cytolytic activity of the host’s serum [20].
All spirochete strains causing Lyme disease have many
varieties of DNA molecules, which replicate as circular
plasmids. These plasmids are called cp32 because of their
circular structure (cp-circular plasmid) and size of about
32 kbp [17, 56]. They are very similar to each other, with
the exception of three loci: one associated with the replication
of plasmid and segregation, and two connected with
encoding the Erp lipoprotein exposed on the surface [23,
49, 56]. A surprising fact is that these plasmids are the genome
of lysogenic bacteriophags which evidently infect all
spirochetes causing Lyme disease [16, 56, 67].
Each element of cp32 includes mono or bicystronic erp
locus, which may differentiate in sequences in individual
plasmids [5, 56]. Erp lipoproteins are from the group of
specifi c proteins synthesized by B. burgdorferi during the
infection of mammal, and in this time their function is the
same as in case of other proteins from this group. It consists
in binding the protein factor H from the host’s serum
during the alternative way of complement activation [2,
3, 30, 35, 57]. Factor H is usually bound by the receptors
on the surface of host’s cells, where it protects these cells
by repressing disintegration of C3 component and protects
it against C3b degradation. It has been suggested that
binding factor H by the Erp and other surface proteins of
B. burgdorferi is a way to protect the pathogen against the
destructive activity of the complement [36].
Members of the cp32 family and erp loci of four isolates
of B. burgdorferi have been described so far [5]. Currently,
it is known that, e.g. strain B31 of B. burgdorferi, includes
10 different cp32 and 17 erp genes. Furthermore, if one bicystronic
locus is present in three identical copies and another
locus, erpH is a natural defect, such bacterium from
B32 strain may synthesize 13 different Erp lipoproteins at
the same time [16, 17]. Babb et al. [3] have shown that
each locus is preceded by highly conservative sequences
of DNA in which a transcription promotor and two different,
separated from each other places occur, which in specifi
c way binds the two different cytoplasmatic proteins of
the bacteria. The same authors have shown that a region
binding the proteins being close to erp promotor, called
operator 2, is indispensable for appropriate regulation of
the transcription of erp gene. Continuing their studies,
Babb et al. [5] have detected an EbfC protein encoded by a
chromosome, which binds a specifi c sequences of DNA in
the 5′ end of all erp loci. The authors conclude that localization
of ebfc gene on the chromosome of B. burgdorferi
suggests that cp32 prophags take part in utilizing proteins
of the host’s cells for their own use, and that EbfC protein
probably plays an additional role in the bacterial cell.
The discussed studies concerned the expression of Erp
proteins in B. burgdorferi during mammal’s infection,
whereas Miller et al. [40] have analyzed the expression of
Erp during the cycle: penetration of tick and infection of
mammals, which showed that bacterium in in vivo conditions
regulates the synthesis of Erp. The bacteria show low
expression of Erp protein in the unfed tick, but when the
infected tick feeds on mouse, B. burgdorferi intensifi es the
production of Erp basically in all cells that penetrated into
mammal. The infected mouse produces antibodies of the
IgM class against all investigated Erp proteins, then there
is a strong response in the form of IgG immunoglobulins.
The latter grow the most intensively in the 11th month of
infection, which suggests the continuation of the exposition
on Erp proteins of the host’s immune system during
even the chronic stage of infection. If uninfected larvae
obtained B. burgdorferi while feeding on mouse, basically
all bacteria transferred into it did not produce Erp proteins,
which according to the authors suggests that the production
is also continued during the infection in mouse. Some time
after translocation of bacteria into larvae, the synthesis of
Erp in their cells is drastically reduced. The expression of
Erp proteins in B. burgdorferi during the infection in mammal
is stable with the hypothetic function of binding factor
H in order to protect the bacteria against the host’s immune
response.
Another protein, called surface protein, binding the
regulator of complement, CRASP-1 (complement regulator-
acquiring surface protein) also binds the protein factors
H and FHL-1 from human serum, and is implicated
in processes associated with the survival of B. burgdorferi
species causing Lyme disease [51]. Cytoplasmatic proteins
control the activation of an alternative way of complement
on the level of C3b component by factor B that binds it.
Additionally, FH and FHL-1 accelerate the disintegration
of C3 convertaze, C3bBb and function as a co-factor for
factor I causing the degradation of C3b [68]. Currently, fi ve
CRASPs, exposed on the surface, isolated from the complement,
proteins binding FH and FHL-1 in it (CRASP-1 and
CRASP-2), or only factor H (CRASP-3, -4, and -5 and Erp
proteins) have been identifi ed [2]. Among CRASP proteins,
the protein CRASP-1 in B. burgdorferi binds the main factors
FH and FHL-1 giving immunity to the complement in
the in vitro culture [35]. The latest studies have shown that
inactivation of gene encoding CRASP-1 in B. burgdorferi
gives results in the serum-sensitive phenotype and addition
of mutated strain with CRASP-1, restores the resistance
Adaptation factors of Borrelia for host and vector 5
for lysis initiated by the complement [12]. These data suggests
that CRASP-1 causes avoiding and/or survival of
spirochetes in the human organism [51], but the expression
of CRASP-1 during the infection in humans is still
under discussion [61]; e.g. studies by McDowell et al. [39]
have shown that the serum of patients with Lyme disease
did not include antigens specifi c for denaturated recombinants
of CRASP-1, tested with the Western blot method.
However, the latest studies of Rossmann et al. [51] have
shown that the serum of patients with Lyme disease was
immunoreactive for undenaturated CRASP-1, investigated
with the ELISA method and immunoblot test, but reactive
for CRASP-1 denaturated in the Western blot. Therefore,
occurring antigens are restrictive for undenaturated structurally
determinants of functionally active proteins, and
the occurred antigens do not interrupt in binding FH by
CRASP-1. The authors conclude that the results, by showing
the expression of CRASP-1 in the immunogenic forms
of spirochetes during the infection of humans, suggest the
engagement of CRASP-1 strategy for avoiding its immune
response by binding FH. In the work by Kraiczy et al. [35],
serum samples from human Lyme disease patients in the
USA and Germany who had a range of disease symptoms,
were examined for the presence of BbCRASP-2-directed
antibodies. Sequences of cspZ genes encoded this protein
were determined for a variety of Borrelial strains of different
genospecies. The results indicate that cspZ sequences
are very conservative among borreliae of Lyme disease,
independently of their geographic distribution, and that antibodies
recognizing BbCRASP-2 are frequently produced
by humans with Lyme disease. In vitro studies of cspZ and
other BbCRASP-encoding genes were also performed to
help elucidate the mechanisms by which BbCRASP levels
are controlled, and seems to be a unique regulatory mechanism
for each class of BbCRASP that result in distinct in
vivo expression profi les [15] .
The vls gene is very essential for the survival
of borreliae in mammals
For the survival of borreliae, the vls gene is most essential,
which encodes surface proteins of 34 kDa. They
consist of two stable components and one variable. The
maintenance of pathogenic species of borreliae, in spite of
active immune response of the host, is partly facilitated by
the specifi c structure of the vls gene. This gene is a complex
consisting of an expressive part, vlsE and contiguous
kit including 11-15 silent vls cassettes [14]. Segments of
the cassettes not subject to expression, recombine with the
vlsE region during host infections of mammals, as a result
they obtain surface protein VlsE in antigenic variance.
VlsE is the surface-exposed protein, on the outer, proteinlipid
membrane of B. burgdorferi spirochetes causing
Lyme disease. During the infection of mammals, but not
during the colonization of tick or in the laboratory culture,
the segments of silent vlsE cassettes randomly recombine
with the expressive vlsE locus, continually creating new
versions of vlsE gene [32, 45, 66].
The effect of conversions of sequences is that variants
of surface VlsE protein change their antigenic properties,
which evidently allow bacteria to avoid rearrangements
– modifi cations of antibodies by the host. It seems to be
an essence of the nature of this gene, because it has been
observed that mutants of these bacteria, devoid of plasmids
with vls, are not able to infect mammals in the long-term
[37, 38, 48, 66], and that vls loci are present in all investigated
spirochetes causing Lyme disease [33, 63]. The
analogical systems of antigenic variants are also associated
with the existing infections with spirochetes of recurrent
typhus [55], in the protozoonosis such as Trypanosoma
spp., and other important pathogens [22].
Borreliae loses genetic material and pathogenicity
during passage
Species from Borrelia genus may lose their pathogenicity
during passaging several times without a selective pressure
for this property [29]. This loss is accompanied by
the loss of genetic material. It is not possible to asses the
range of this loss based only on data from PFGE or PCR,
especially if the whole genome sequence is not known
[29]. Therefore, Glöckner et al. [29] have sequenced and
analysed plasmids from B. garinii strains coming from low
and high passage, and also B. afzelii strains, in order to
describe all sequence differences and to detect the cause
of pathogenicity loss. It has occurred that not the whole
plasmid but only a part was lost during the passage in PBi
strain of B. garinii. The rest of the plasmid was saved by
accident, or because it was important. The lost part consisted
of cassettes of vls gene, whose frequent exchanges
are engaged in the escape of B. burgdorferi from the host’s
response. Perhaps this selective loss may be caused by the
repair mistakes in the locus after recombination. The authors
have found that the lack of selective pressure allows
this clone with cut plasmid to prosper in culture. They conclude
that in order for the entire culture to lose the locus,
cassette switching errors must be frequent or the mutated
clone has a substantial growth advantage.
In the PKo strain of B. afzelii from the high passage, the
whole plasmid was absent. This plasmid also included the
vls genes, which may be deduced from the presence of a
few vls cassette derived readouts. The loss of the whole
plasmid will happen if plasmids are not correctly divided
during the division between daughter cells. So far, we have
discovered that two sequences, in the collection of the fragments,
come from DNA of a high passage strain, which
indicates that this was a two-stage process of losing the
plasmid. According to this scenario, the group of the vls
genes was lost, like as the fi rst one, as in B. garinii PBi, and
then the remainder of the plasmid.
The interesting fact is that in a tick-vector, the locus of
the vls gene seems to be muffl ed, therefore it is stable in
6 Skotarczak B
this host. This explains why this accidental loss does not
occur in nature.
The authors hypothesize that this additional loss happened
only by coincidence, caused by the unessential nature
of the plasmidial remains. This makes the vls locus
a visible but sensitive factor of the successful prospering
of Borrelia species in the individual hosts – vertebrates.
Further studies on the additional strains from low and high
passages should reveal if the loss of the genome cassettes
of the vls is a leading or single event causing the loss of
pathogenicity.
Quorum sensing
The list of known species of bacteria that use quorum
sensing mechanism to regulate genes’ expression, which
enables a simultaneous response of the whole population to
the environmental changes, has grown recently.
For the fi rst time, the quorum sensing phenomenon was
described in the marine bacteria Vibrio fisher which have
an ability of bioluminescence. They occur in sea water and
also in the luminous organ of cuttlefish. Their concentration
in water is low and they do not emit light. But when their
concentration in the luminous organ of cuttlefish grows up
to 109 cells/ml, all bacteria start to emit light energy [20].
The mechanism of this phenomenon lies in the fact that
bacteria excrete a specifi c factor called autoinducer AI-2.
The excreted outside the cells autoinducer penetrates the
cells once more and when its concentration in the cell increases
to the proper level, it raises the expression of some
genes being inactive. It has been shown that other species
of marine bacterium, V. harveyi use AI-2 in the quorum
sensing mechanism to regulate bioluminescence, and AI-2
induces bacteria to produce light independent of the autoinducer
source [8].
Two general types of autoinducers have been identified in
bacteria. The first type is specific for the species producing
it, for example, homoserine lactones or certain polypeptides,
and the second type, autoinductor-2 (AI-2), which well conserved
across species. AI-2 is produced from methionine
and ATP through a fi ve-step process catalyzed by S-adenosylmethionine
synthetase (MetK), a methyltransferase,
S-adenosylhomocysteine/5-methylthioribose nucleosidase
(Pfs), and LuxS [18]. The fi nal step involves an apparently
spontaneous cyclization of the LuxS product (4,5-dihydroxy-
2,3-pentanedione) with borate to produce AI-2. The
fi rst three enzymes in this course appear to be important for
bacterial survival [60]. Since AI-2-mediated quorum sensing
is implicated in the regulation of virulence properties in
a wide variety of pathogenic bacteria, LuxS has also been an
essential enzyme of many bacteria in nature [58].
In order to understand the pathogenic properties of infectious
agents such as B. burgdorferi, a valuable step is
the defi ning of the metabolic capabilities and regulatory
mechanisms controlling gene expression [4]. Stevenson
and Babb [58] have shown that B. burgdorferi encodes
a functional LuxS enzyme enabling it to synthesize AI-2.
What is more, addition of this autoinducer to the culture
of B. burgdorferi has profound effects on the expression
levels of many bacterial proteins. This indicates the importance
of this quorum sensing system in the regulation of
Borrelial protein expression.
In the studies of Hübner et al. [31], the luxS gene was expressed
by Borrelia burgdorferi strain 297 cultured in vitro,
or in dialysis membrane chambers implanted in rat peritoneal
cavities. Although the Borrelial luxS gene functionally
complemented a LuxS defi ciency in Escherichia coli DH5α,
AI-2-like activity was not detected within B. burgdorferi
culture supernatants or concentrated cell lysates. Finally,
a luxS-defi cient mutant of B. burgdorferi was infectious at
wild-type levels when inoculated into mice, indicating that
the expression of luxS is probably not required for infectivity
but, at the very least, is not essential for mammalian host
adaptation. These fi ndings may also challenge the notion
that a LuxS/AI-2 quorum sensing system is operative in
B. burgdorferi. However, the studies of von Lackum et al.
[62] demonstrated that B. burgdorferi encodes functional
Pfs and LuxS enzymes for the breakdown of toxic products
of methylation reactions. According to these observations,
B. burgdorferi was shown to synthesize the fi nal product,
4,5-dihydroxy-2,3-pentanedione (DPD) during laboratory
cultivation. DPD undergoes spontaneous rearrangements
to produce a class of pheromones collectively named autoinducer
2 (AI-2). The addition of in vitro-synthesized
DPD to the culture of B. burgdorferi manifested in differential
expression of a distinct subset of proteins, including
the outer surface lipoprotein VlsE. Although many bacteria
for regeneration of methionine can utilize the other LuxS
product, homocysteine, B. burgdorferi did not show such
an ability. It is hypothesized that B. burgdorferi produces
LuxS for the express purpose of synthesizing DPD, and
utilizes a form of that molecule as an AI-2 pheromone to
control gene expression [4]. Whereas the studies of Riley
et al. [50] have demonstrated that single operon encoding
four enzymes occurs in B. burgdorferi, two of them are associated
with the synthesis of DPD, one was found only in
borreliae causing Lyme disease and is an activated-methyl
donor, and the fourth is the gene encoding phosphohydrolase.
All four genes have shown that coexpression and high
metabolic activity was accompanied by the growth of the
cell level of methyl donor, increased the detoxication of
methylation products, and growth of the DPD synthesis.
Therefore, the authors conclude that the production of DPD
is directly correlated with the level of cell metabolism, and
perhaps functions as an extracellular signal and/or intercellular
for bacteria.
SUMMARY
This review focuses some strategies to adopt to the
vertebrate hosts and arthropod vector that B. burgdorferi
have developed despite its relatively small genome.
Adaptation factors of Borrelia for host and vector 7
Special properties of B. burgdorferii are associated with
large number of plasmids, rare in other bacteria, which
show a high variability as a result of recombination with
each other. Changes in the synthesis of outer proteins,
Osp are the strategy of borreliae in order to avoid the destructive
effect of the immune system of the host. Then,
by colonizing tissues, they initiate production of Erp and
CRASP proteins, which bind the regulators of complement
and repress a cytolytic effect of the host’s serum. Some
evidence indicates that Lyme disease spirochetes use quorum
sensing as a mechanism to control the protein expression.
The quorum sensing phenomenon is associated with
autoinducer AI-2 and i.a. the LuxS protein is commited to
its synthesis. The review of results of the expression studies
of luxS gene shows that molecular mechanisms of this
phenomenon in B. burgdorferi are still only fragmentarily
known. Continuation of the quorum sensing studies, as
well as other mechanisms of controlling the expression of
genes in B. burgdorferi, will help to understand the pathogenic
properties of this bacterium, and to improve the construction
of vaccines and the therapy of Lyme disease.
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REVIEW ARTICLES AAEM 
http://www.aaem.pl/pdf/16001.pdf   .
ADAPTATION FACTORS OF BORRELIA FOR HOST AND VECTOR
Bogumiła Skotarczak
Department of Genetics, University of Szczecin, Szczecin, Poland
Skotarczak B: Adaptation factors of Borrelia for host and vector. Ann Agric Environ Med
2009, 16, 1-8.