Genetic manipulation of spirochetes--light at the end of the tunnel

Genetic Manipulation of Borrelia

Genetic studies in Borrelia require special consideration of the highly segmented genome, complex growth requirements and evolutionary distance of spirochetes from other genetically tractable bacteria. Despite these challenges, a robust molecular genetic toolbox has been constructed to investigate the biology and pathogenic potential of these important human pathogens. In this review we summarize the tools and techniques that are currently available for the genetic manipulation of Borrelia, including the relapsing fever spirochetes, viewing them in the context of their utility and shortcomings. Our primary objective is to help researchers discern what is feasible and what is not practical when thinking about potential genetic experiments in Borrelia. We have summarized published methods and highlighted their critical elements, but we are not providing detailed protocols. Although many advances have been made since B. burgdorferi was first transformed over 25 years ago, some standard genetic tools remain elusive for Borrelia. We mention these limitations and why they persist, if known. We hope to encourage investigators to explore what might be possible, in addition to optimizing what currently can be achieved, through genetic manipulation of Borrelia.

Borrelia burgdorferi adhesins identified using in vivo phage display

Borrelia burgdorferi, the agent of Lyme disease, disseminates from the site of deposition by Ixodes ticks to cause systemic infection. Dissemination occurs through the circulation and through tissue matrices, but the B. burgdorferi molecules that mediate interactions with the endothelium in vivo have not yet been identified. In vivo selection of filamentous phage expressing B. burgdorferi protein fragments on the phage surface identified several new candidate adhesins, and verified the activity of one adhesin that had been previously characterized in vitro. P66, a B. burgdorferi ligand for beta(3)-chain integrins, OspC, a protein that is essential for the establishment of infection in mammals, and Vls, a protein that undergoes antigenic variation in the mammal, were all selected for binding to the murine endothelium in vivo. Additional B. burgdorferi proteins for which no functions have been identified, including all four members of the OspF family and BmpD, were identified as candidate adhesins. The use of in vivo phage display is one approach to the identification of adhesins in pathogenic bacteria that are not easily grown in the laboratory, or for which genetic manipulations are not straightforward.

Enigmatic dual symbiosis in the excretory organ of Nautilus macromphalus (Cephalopoda: Nautiloidea)

Symbiosis is an important driving force in metazoan evolution and the study of ancient lineages can provide an insight into the influence of symbiotic associations on morphological and physiological adaptations. In the 'living fossil' Nautilus, bacterial associations are found in the highly specialized pericardial appendage. This organ is responsible for most of the excretory processes (ultrafiltration, reabsorption and secretion) and secretes an acidic ammonia-rich excretory fluid. In this study, we show that Nautilus macromphalus pericardial appendages harbour a high density of a beta-proteobacterium and a coccoid spirochaete using transmission electron microscopy, comparative 16S rRNA sequence analysis and fluorescence in situ hybridization (FISH). These two bacterial phylotypes are phylogenetically distant from any known bacteria, with ammonia-oxidizing bacteria as the closest relatives of the beta-proteobacterium (above or equal to 87.5% sequence similarity) and marine Spirochaeta species as the closest relatives of the spirochaete (above or equal to 89.8% sequence similarity), and appear to be specific to Nautilus. FISH analyses showed that the symbionts occur in the baso-medial region of the pericardial villi where ultrafiltration and reabsorption processes take place, suggesting a symbiotic contribution to the excretory metabolism.

Lyme borreliosis in 2005, 30 years after initial observations in Lyme Connecticut

Nearly 100 years ago, Afzelius described a patient with an expanding skin lesion, called erythema migrans, which is now known to be the initial skin manifestation of Lyme borreliosis. Approximately 70 years later, in 1976, epidemiologic evaluation of a cluster of children with arthritis in Lyme, Connecticut led to a complete description of the infection. During the subsequent years, investigators in a number of countries have made remarkable strides in the elucidation of this tick-borne spirochetal infection. The purpose of this review is to discuss the current status of Lyme borreliosis, including areas in which knowledge of the infection is still incomplete.

Borrelia burgdorferi ftsZ plays a role in cell division

ftsZ is essential for cell division in many microorganisms. In Escherichia coli and Bacillus subtilis, FtsZ plays a role in ring formation at the leading edge of the cell division septum. An ftsZ homologue is present in the Borrelia burgdorferi genome (ftsZ(Bbu)). Its gene product (FtsZ(Bbu)) is strongly homologous to other bacterial FtsZ proteins, but its function has not been established. Because loss-of-function mutants of ftsZ(Bbu) might be lethal, the tetR/tetO system was adapted for regulated control of this gene in B. burgdorferi. Sixty-two nucleotides of an ftsZ(Bbu) antisense DNA sequence under the control of a tetracycline-responsive modified hybrid borrelial promoter were cloned into pKFSS1. This construct was electroporated into a B. burgdorferi host strain carrying a chromosomally located tetR under the control of the B. burgdorferi flaB promoter. After induction by anhydrotetracycline, expression of antisense ftsZ RNA resulted in generation of filamentous B. burgdorferi that were unable to divide and grew more slowly than uninduced cells. To determine whether FtsZ(Bbu) could interfere with the function of E. coli FtsZ, ftsZ(Bbu) was amplified from chromosomal DNA and placed under the control of the tetracycline-regulated hybrid promoter. After introduction of the construct into E. coli and induction with anhydrotetracycline, overexpression of ftsZ(Bbu) generated a filamentous phenotype. This suggested interference of ftsZ(Bbu) with E. coli FtsZ function and confirmed the role of ftsZ(Bbu) in cell division. This is the first report of the generation of a B. burgdorferi conditional lethal mutant equivalent by tetracycline-controlled expression of antisense RNA.

Genetic and physiological characterization of 23S rRNA and ftsJ mutants of Borrelia burgdorferi isolated by mariner transposition

Borrelia burgdorferi contains one 16S rRNA gene and two tandem sets of 23S and 5S rRNA genes located in a single chromosomal region. This unusual rRNA gene organization has been speculated to be involved in the slow growth of this organism. Because we were repeatedly unable to isolate a 23S ribosomal mutant in B. burgdorferi by allelic exchange, we developed a transposition mutagenesis system for this bacterium. To this end, Himar1 transposase is expressed in B. burgdorferi from a resident plasmid containing an erythromycin resistance marker, and this strain is then electroporated with suicide plasmids containing mariner transposons and kanamycin resistance genes expressible in B. burgdorferi. This system permitted us to generate hundreds of erythromycin/kanamycin-resistant B. burgdorferi clones with each of three suicide plasmids. DNA sequencing of several kanamycin-resistant clones generated with one of the suicide plasmids showed stable and random insertion of the transposon into the B. burgdorferi chromosomal and plasmid genome. One mutant was inactivated in rrlA (23S rRNA), another in ftsJ (rrmJ). rrlA disruption had no effect on growth rate under a wide range of culture conditions, but disruption of ftsJ interfered significantly with growth rate and bacterial morphology. These data show it is possible to isolate random and stable B. burgdorferi transposition mutants for physiological analysis of this pathogenic spirochete.

Analysis of the ospC regulatory element controlled by the RpoN-RpoS regulatory pathway in Borrelia burgdorferi

Outer surface lipoprotein C (OspC) is a key virulence factor of Borrelia burgdorferi. ospC is differentially regulated during borrelial transmission from ticks to rodents, and such regulation is essential for maintaining the spirochete in its natural enzootic cycle. Recently, we showed that the expression of ospC in B. burgdorferi is governed by a novel alternative sigma factor regulatory network, the RpoN-RpoS pathway. However, the precise mechanism by which the RpoN-RpoS pathway controls ospC expression has been unclear. In particular, there has been uncertainty regarding whether ospC is controlled directly by RpoS (sigma(s)) or indirectly through a transactivator (induced by RpoS). Using deletion analyses and genetic complementation in an OspC-deficient mutant of B. burgdorferi, we analyzed the cis element(s) required for the expression of ospC in its native borrelial background. Two highly conserved upstream inverted repeat elements, previously implicated in ospC regulation, were not required for ospC expression in B. burgdorferi. Using similar approaches, a minimal promoter that contained a canonical -35/-10 sequence necessary and sufficient for sigma(s)-dependent regulation of ospC was identified. Further, targeted mutagenesis of a C at position -15 within the extended -10 region of ospC, which is postulated to function like the strategic C residue important for Esigma(s) binding in Escherichia coli, abolished ospC expression. The minimal ospC promoter also was responsive to coumermycin A(1), further supporting its sigma(s) character. The combined data constitute a body of evidence that the RpoN-RpoS regulatory network controls ospC expression by direct binding of sigma(s) to a sigma(s)-dependent promoter of ospC. The implication of our findings to understanding how B. burgdorferi differentially regulates ospC and other ospC-like genes via the RpoN-RpoS regulatory pathway is discussed.

bptA (bbe16) is essential for the persistence of the Lyme disease spirochete, Borrelia burgdorferi, in its natural tick vector

Borrelia burgdorferi (Bb), the agent of Lyme disease, is a zoonotic spirochetal bacterium that depends on arthropod (Ixodes ticks) and mammalian (rodent) hosts for its persistence in nature. The quest to identify borrelial genes responsible for Bb's parasitic dependence on these two diverse hosts has been hampered by limitations in the ability to genetically manipulate virulent strains of Bb. Despite this constraint, we report herein the inactivation and genetic complementation of a linear plasmid-25-encoded gene (bbe16) to assess its role in the virulence, pathogenesis, and survival of Bb during its natural life cycle. bbe16 was found to potentiate the virulence of Bb in the murine model of Lyme borreliosis and was essential for the persistence of Bb in Ixodes scapularis ticks. As such, we have renamed bbe16 a gene encoding borrelial persistence in ticks (bpt)A. Although protease accessibility experiments suggested that BptA as a putative lipoprotein is surface-exposed on the outer membrane of Bb, the molecular mechanism(s) by which BptA promotes Bb persistence within its tick vector remains to be elucidated. BptA also was shown to be highly conserved (>88% similarity and >74% identity at the deduced amino acid levels) in all Bb sensu lato strains tested, suggesting that BptA may be widely used by Lyme borreliosis spirochetes for persistence in nature. Given Bb's absolute dependence on and intimate association with its arthropod and mammalian hosts, BptA should be considered a virulence factor critical for Bb's overall parasitic strategy.

The emergence of Lyme disease

Since its identification nearly 30 years ago, Lyme disease has continued to spread, and there have been increasing numbers of cases in the northeastern and north central US. The Lyme disease agent, Borrelia burgdorferi, causes infection by migration through tissues, adhesion to host cells, and evasion of immune clearance. Both innate and adaptive immune responses, especially macrophage- and antibody-mediated killing, are required for optimal control of the infection and spirochetal eradication. Ecological conditions favorable to the disease, and the challenge of prevention, predict that Lyme disease will be a continuing public health concern.

Essential role for OspA/B in the life cycle of the Lyme disease spirochete

The molecular basis of how Borrelia burgdorferi (Bb), the Lyme disease spirochete, maintains itself in nature via a complex life cycle in ticks and mammals is poorly understood. Outer surface (lipo)protein A (OspA) of Bb has been the most intensively studied of all borrelial molecular constituents, and hence, much has been speculated about the potential role(s) of OspA in the life cycle of Bb. However, the precise function of OspA (along with that of its close relative and operonic partner, outer surface [lipo]protein B [OspB]) heretofore has not been directly determined, due primarily to the inability to generate an OspA/B-deficient mutant from a virulent strain of Bb. In this study, we created an OspA/B-deficient mutant of an infectious human isolate of Bb (strain 297) and found that OspA/B function was not required for either Bb infection of mice or accompanying tissue pathology. However, OspA/B function was essential for Bb colonization of and survival within tick midguts, events crucial for sustaining Bb in its natural enzootic life cycle.

Genetics of motility and chemotaxis of a fascinating group of bacteria: the spirochetes

Spirochetes are a medically important and ecologically significant group of motile bacteria with a distinct morphology. Outermost is a membrane sheath, and within this sheath is the protoplasmic cell cylinder and subterminally attached periplasmic flagella. Here we address specific and unique aspects of their motility and chemotaxis. For spirochetes, translational motility requires asymmetrical rotation of the two internally located flagellar bundles. Consequently, they have swimming modalities that are more complex than the well-studied paradigms. In addition, coordinated flagellar rotation likely involves an efficient and novel signaling mechanism. This signal would be transmitted over the length of the cell, which in some cases is over 100-fold greater than the cell diameter. Finally, many spirochetes, including Treponema, Borrelia, and Leptospira, are highly invasive pathogens. Motility is likely to play a major role in the disease process. This review summarizes the progress in the genetics of motility and chemotaxis of spirochetes, and points to new directions for future experimentation.

Targeted mutation of the outer membrane protein P66 disrupts attachment of the Lyme disease agent, Borrelia burgdorferi, to integrin alphavbeta3

Borrelia burgdorferi, the agent of Lyme disease, expresses several adhesion molecules that are probably required for initial establishment of infection in mammalian hosts, and for colonization of various tissues within the host. The B. burgdorferi outer membrane protein P66 was previously identified as a ligand for beta3-chain integrins by using a variety of biochemical approaches. Although the earlier data suggested that P66 is an adhesin that mediates B. burgdorferi attachment to beta3-chain integrins, lack of genetic systems in B. burgdorferi precluded definitive demonstration of a role for P66 in beta3 integrin attachment by intact borreliae. Recent advances in the genetic manipulation of B. burgdorferi have now made possible the targeted disruption of the p66 gene. Mutants in p66 show dramatically reduced attachment to integrin alphavbeta3. This is, to our knowledge, the first description of the targeted disruption of a candidate B. burgdorferi virulence factor with a known biochemical function that can be quantified, and demonstrates the importance of B. burgdorferi P66 in the attachment of this pathogenic spirochete to a human cell-surface receptor.

Novel antibiotic-resistance markers in pGK12-derived vectors for Borrelia burgdorferi

Extension of molecular genetics studies in Borrelia burgdorferi has been hampered by a lack of a variety of antibiotic resistance selective markers. Such markers are critical for isolation of B. burgdorferi strains with multiple mutants, for complementation with different cloning vectors, and for methods such as negative selection and reporter genes. To remedy this lack, resistance to various antibiotics of non-infectious (B31, 297) and infectious (N40) B. burgdorferi strains was examined and vectors incorporating appropriate antibiotic resistance genes as selective markers were developed. Minimal inhibitory concentrations for growth of B. burgdorferi on plates and in liquid media for aminoglycosides (kanamycin, gentamycin, sisomycin, amikacin, spectinomycin, neomycin), macrolides-lincosamids (erythromycin, lincomycin), coumarin derivatives (coumermycin A(1), novobiocin), glycopeptides (vancomycin, ristocetin), peptides (bacitracin, cycloserine), and chloramphenicol were found to differ significantly. There were also striking differences in resistance to these antibiotics between non-infectious and infectious B. burgdorferi strains. Antibiotic-resistance genes aph(3')-IIIa from Streptococcus faecalis, aad9 from Staphylococcus aureus Tn554, linA' from Staphylococcus aureus, and aac(3)-VIa from Enterobacter cloacae (conferring resistance to kanamycin, spectinomycin, lincomycin, and gentamycin/sisomycin, respectively) were subcloned either with their own promoters or under the control of the B. burgdorferi flaB promoter into pGK12 or its derivative pED1 to develop new cloning vectors for B. burgdorferi with the rationale that the absence of homologous regions between derived recombinant plasmids lacking the flaB promoter and the B. burgdorferi genome would permit avoidance of possible recombination with target DNA. Resistance to the corresponding antibiotic was conferred by vectors containing aph(3')-IIIa, aad9, linA' or aac(3)-VIa whether under the control of their own promoters or under the control of the flaB promoter. We conclude that these markers can be used for genetic study of B. burgdorferi and suggest they will be an important addition to the previously used coumermycin A(1), erythromycin and kanamycin in these studies.

Erythromycin resistance in Borrelia burgdorferi

Susceptibility testing of laboratory strains and clinical isolates of Borrelia burgdorferi indicates that resistance to erythromycin is present in them. Evaluation of the MICs, minimal bactericidal concentrations, and kinetics of bacterial killing of erythromycin suggests that this resistance is increased by preexposure to the antibiotic, is dependent on inoculum size, and may be the result of selection of subpopulations of bacterial cells with increased resistance.

Modulation of Borrelia burgdorferi stringent response and gene expression during extracellular growth with tick cells

Borrelia burgdorferi N40 multiplied extracellularly when it was cocultured with tick cells in L15BS medium, a medium which by itself did not support B. burgdorferi N40 growth. Growth of B. burgdorferi N40 in the presence of tick cells was associated with decreased production of (p)ppGpp, the stringent response global regulator, a fourfold decrease in relA/spoT mRNA, an eightfold net decrease in bmpD mRNA, and a fourfold increase in rpsL-bmpD mRNA compared to growth of B. burgdorferi in BSK-H medium. As a result, the polycistronic rpsL-bmpD mRNA level increased from 3 to 100% of the total bmpD message. These observations demonstrate that there are reciprocal interactions between B. burgdorferi and tick cells in vitro and indicate that the starvation-associated stringent response mediated by (p)ppGpp present in B. burgdorferi growing in BSK-H medium is ameliorated in B. burgdorferi growing in coculture with tick cell lines. These results suggest that this system can provide a useful model for identifying genes controlling interactions of B. burgdorferi with tick cells in vitro when it is coupled with genetic methods to isolate and complement B. burgdorferi mutants.

Complementation of a nonmotile flaB mutant of Borrelia burgdorferi by chromosomal integration of a plasmid containing a wild-type flaB allele

With the recent identification of antibiotic resistance phenotypes, the use of reporter genes, the isolation of null mutants by insertional inactivation, and the development of extrachromosomal cloning vectors, genetic analysis of Borrelia burgdorferi is becoming a reality. A previously described nonmotile, rod-shaped, kanamycin-resistant B. burgdorferi flaB::Km null mutant was complemented by electroporation with the erythromycin resistance plasmid pED3 (a pGK12 derivative) containing the wild-type flaB sequence and 366 bp upstream from its initiation codon. The resulting MS17 clone possessed erythromycin and kanamycin resistance, flat-wave morphology, and microscopic and macroscopic motility. Several other electroporations with plasmids containing wild-type flaB and various lengths (198, 366, or 762 bp) of sequence upstream from the flaB gene starting codon did not lead to functional restoration of the nonmotile flaB null mutant. DNA hybridization, PCR analysis, and sequencing indicated that the wild-type flaB gene in nonmotile clones was present in the introduced extrachromosomal plasmids, while the motile MS17 clone was a merodiploid containing single tandem chromosomal copies of mutated flaB::Km and wild-type flaB with a 366-bp sequence upstream from its starting codon. Complementation was thus achieved only when wild-type flaB was inserted into the borrelial chromosome. Several possible mechanisms for the failure of complementation for extrachromosomally located flaB are discussed.