Environmentally responsive synthesis of surface proteins represents a hallmark of the infectious cycle of the Lyme disease agent, Borrelia burgdorferi. Here we created and analyzed a B. burgdorferi mutant lacking outer-surface protein C (OspC), an abundant Osp that spirochetes normally synthesize in the tick vector during the blood meal and down-regulate after transmission to the mammal. We demonstrate that B. burgdorferi strictly requires OspC to infect mice but not to localize or migrate appropriately in the tick. The induction of a spirochetal virulence factor preceding the time and host in which it is required demonstrates a developmental sequence for transmission of this arthropod-borne pathogen.

A major obstacle to studying the functions of particular gene products in the mouse-tick infectious cycle of Borrelia burgdorferi has been an inability to knock out genes in pathogenic strains. Here, we investigated conditions for site-directed mutagenesis in B31 MI, the low-passage-number, infectious B. burgdorferi strain whose genome was sequenced. We inactivated several plasmid and chromosomal genes in B31 MI and determined that clones carrying these mutations were not infectious for mice. However, we found extensive heterogeneity among clones and mutants derived from B31 MI based on colony phenotype, growth rate, plasmid content, protein profile, and transformability. Significantly, several B31 MI clones that were not subjected to mutagenesis but that lacked particular plasmids also exhibited defects at various stages in the infectious cycle. Therefore, the high degree of clonal polymorphism within B31 MI complicates the assessment of the contributions of individual genes to the observed phenotypes of the mutants. Our results indicate that B31 MI is not an appropriate strain background for genetic studies in infectious B. burgdorferi, and a well-defined isogenic clone is a prerequisite for targeted mutagenesis. To this end, we derived several wild-type clones from B31 MI that were infectious for mice, and gene inactivation was successful in one of these clones. Due to the instability of the genome with in vitro propagation, careful monitoring of plasmid content of derived mutants and complementation of inactivated genes will be crucial components of genetic studies with this pathogen.Lyme disease is caused by Borrelia burgdorferi, a spirochete transmitted by ticks of the genus Ixodes and maintained within an enzootic cycle between the tick vector and mammalian hosts, most importantly small rodents (7,11,19). The clinical manifestations of this zoonosis can include a multisystem disorder affecting skin and joints and the nervous, lymphoreticular, and cardiovascular systems (39,40).The organization of the B. burgdorferi genome is unique among bacteria in that the genome is composed of a linear chromosome and a large number of linear and circular plasmids (8, 14). The complete genome sequence of an infectious B. burgdorferi isolate, the type strain B31, identified 21 linear and circular plasmids (8). In vitro propagation of B. burgdorferi can lead to plasmid loss and concurrent loss of infectivity for mice (3,21,22,30,33). Although increasing evidence suggests that certain Borrelia plasmids are important for infection in mice (18,25), this hypothesis has not been experimentally verified, and the roles of most plasmid-encoded genes in the infectious cycle are unknown. A number of plasmid-and chromosomally encoded genes have been inactivated in the highpassage-number, noninfectious clone B31-A (4,5,12,17,20,43,45,46), but gene inactivation in a low-passage-number, infectious strain background has not been reported.Here, we investigate conditions for site-directed mutagenesis in B31 MI, the low-passage-number, inf...

This study demonstrates a strict temporal requirement for a virulence determinant of the Lyme disease spirochete Borrelia burgdorferi during a unique point in its natural infection cycle, which alternates between ticks and small mammals. OspC is a major surface protein produced by B. burgdorferi when infected ticks feed but whose synthesis decreases after transmission to a mammalian host. We have previously shown that spirochetes lacking OspC are competent to replicate in and migrate to the salivary glands of the tick vector but do not infect mice. Here we assessed the timing of the requirement for OspC by using an ospC mutant complemented with an unstable copy of the ospC gene and show that B. burgdorferi's requirement for OspC is specific to the mammal and limited to a critical early stage of mammalian infection. By using this unique system, we found that most bacterial reisolates from mice persistently infected with the initially complemented ospC mutant strain no longer carried the wild-type copy of ospC. Such spirochetes were acquired by feeding ticks and migrated to the tick salivary glands during subsequent feeding. Despite normal behavior in ticks, these ospC mutant spirochetes did not infect naive mice. ospC mutant spirochetes from persistently infected mice also failed to infect naive mice by tissue transplantation. We conclude that OspC is indispensable for establishing infection by B. burgdorferi in mammals but is not required at any other point of the mouse-tick infection cycle.

Previous studies have shown that a 54 -S cascade regulates the expression of a few key lipoproteins in Borrelia burgdorferi, the agent of Lyme disease. Here, we demonstrate that these sigma factors, both together and independently, regulate a much more extensive number of genes and cellular processes. Microarray analyses of 54 and S mutant strains identified 305 genes regulated by 54 and 145 regulated by S , whereas the 54 -S regulatory cascade appears to control 48 genes in B. burgdorferi. In silico analyses revealed that nearly 80% of genes with altered expression in the 54 mutant were linked to potential 54 -dependent promoters. Many 54 -regulated genes are expressed in vivo, and through genetic complementation of the mutant, we demonstrated that 54 was required by B. burgdorferi to infect mammals. Surprisingly, 54 mutants were able to infect Ixodes scapularis ticks and be maintained for at least 24 wk after infection, suggesting the 54 -S regulatory network was not involved in long-term survival in ticks. However, 54 mutants did not enter the salivary glands during tick feeding, indicating that 54 -regulated genes were involved in the transmission process.infectivity ͉ microarray ͉ Lyme ͉ transcription

SummaryBorrelia burgdorferi contains abundant circular and linear plasmids, but the mechanism of replication of these extrachromosomal elements is unknown. A B. burgdorferi 9 kb circular plasmid (cp9) was amplified in its entirety by the polymerase chain reaction and used to construct a shuttle vector that replicates in Escherichia coli and B. burgdorferi. A 3.3 kb region of cp9 containing three open reading frames was used to construct a smaller shuttle vector, designated pBSV2. This vector was stably maintained in B. burgdorferi, indicating that all elements necessary for autonomous replication are probably located on this 3.3 kb fragment. A noninfectious B. burgdorferi strain was efficiently transformed by pBSV2. Additionally, infectious B. burgdorferi was also successfully transformed by pBSV2, indicating that infectious strains of this important human pathogen can now be genetically manipulated.

Lyme disease is the most commonly reported vector-borne disease in North America and Europe, yet we know little about which components of the causative agent, Borrelia burgdorferi, are critical for infection or virulence. Molecular genetics has provided a powerful means by which to address these topics in other bacterial pathogens. Certain features of B. burgdorferi have hampered the development of an effective system of genetic analysis, but basic tools are now available and their application has begun to provide information about the identities and roles of key bacterial components in both the tick vector and the mammalian host. Increased genetic analysis of B. burgdorferi should advance our understanding of the infectious cycle and the pathogenesis of Lyme disease.

The spirochete Borrelia burgdorferi is a tick-borne obligate parasite whose normal reservoir is a variety of small mammals [1]. Whereas infection of these natural hosts does not lead to disease, infection of humans can result in Lyme disease, as a consequence of the human immunopathological response to B. burgdorferi [2,3]. Consistent with the pathogenesis of Lyme disease, bacterial products that allow B. burgdorferi to replicate and survive, rather than true "virulence factors," appear to be primarily what is required for the bacterium to cause disease in a susceptible host. In support of this idea, the genome sequence of B31, the type strain of B. burgdorferi sensu stricto [4,5], revealed that the bacterium lacks factors common to many bacterial pathogens, such as lipopolysaccharide, toxins, and specialized secretion systems. In this chapter, we will describe the basic biology of B. burgdorferi, and some of the bacterial components required to infect and survive in the mammalian and tick hosts.

In this report we describe two distinct approaches to develop new antibiotic resistance cassettes that allow for efficient selection of Borrelia burgdorferi transformants. The first approach utilizes fusions of borrelial flagellar promoters to antibiotic resistance markers from other bacteria. The aacC1 gene, which encodes a gentamicin acetyltransferase, conferred a high level of gentamicin resistance in B. burgdorferi when expressed from these promoters. No cross-resistance occurred between this cassette and the kanamycin resistance cassette, which was previously developed in an analogous fashion. A second and different approach was taken to develop an efficient selectable marker that confers resistance to the antibiotic coumermycin A1. A synthetic gene was designed from the gyrB301 allele of the coumermycin-resistant B. burgdorferi strain B31-NGR by altering the coding sequence at the wobble position. The resulting gene, gyrB_syn, encodes a protein identical to the product of gyrB301, but the genes share only 66% nucleotide identity. The nucleotide sequence of gyrB_synis sufficiently divergent from the endogenous B. burgdorferigyrB gene to prevent recombination between them. The cassettes described in this paper improve our repertoire of genetic tools in B. burgdorferi. These studies also provide insight into parameters governing recombination and gene expression in B. burgdorferi.