Molecular Genetic Basis of the Infectious Cycle of Borrelia burgdorferi

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Lyme disease is the most common tick-borne illness in the United States and Europe. It is caused by Borrelia burgdorferi, a bacterial pathogen that is maintained in nature in a zoonotic cycle between various species of small mammals and an ixodid tick vector. A hallmark of the Lyme disease spirochete is its unusual, segmented genome, comprising a linear chromosome and approximately 20 linear and circular plasmids. An increasing body of data demonstrates that plasmid-encoded functions are critical for successful adaptation of B. burgdorferi to the different environments that the spirochete encounters during its natural infectious cycle. We have developed genetic tools to investigate basic aspects of the unusual genomic organization, cellular structure and metabolism of B. burgdorferi. We have extended this investigation to an in vivo setting with an experimental system that closely mimics the natural arthropod vector/rodent host infectious cycle. Through an understanding of the basic molecular biology of the organism, we hope to gain insight into the infectious strategy utilized by this significant vector-borne pathogen and thereby facilitate efforts to prevent, diagnose and treat Lyme disease. The alternative sigma factor RpoS plays a central role in a critical adaptive response of the Lyme disease spirochete that is induced during tick feeding and prepares the spirochete for infection of the vertebrate host. Previous work from our lab identified the plasmid-encoded BBD18 protein as a negative regulator of RpoS, but inactivation of the bbd18 gene in wild-type spirochetes was never achieved. We utilized a conditional bbd18 mutant, genome-wide transcriptomic, metabolomic, and protein profiling, and the first engineered displacement of all cp32 prophage plasmids, to investigate the lethal phenotype that accompanies BBD18 depletion and unmodulated production of RpoS in infectious B. burgdorferi. Additionally, we conducted in vivo studies to identify the precise stage and location during the mouse-tick infectious cycle at which spirochetes require BBD18 for survival. As a result of these experiments, we provide the first report of a link between transducing phage and the RpoS-dependent adaptive response that Lyme disease spirochete undergo during tick feeding. We infer that the multipartite structure of the Borrelia genome facilitates phage-mediated genetic exchange and reassortment, and thereby fosters the level of diversity that is required for spirochete maintenance in the natural enzootic cycle. NOTE: The Principal Investigator retired in April 2023 and this project will be terminated/inactivated.