The burgeoning molecular genetics of the Lyme disease spirochaete

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Abstract
Borrelia burgdorferi, the spirochaete that causes Lyme disease, is a fastidious, slow-growing bacterium. This spirochaete has a complex genome composed of multiple linear and circular plasmids, in addition to a linear chromosome. The B. burgdorferi natural life cycle alternates between ticks and small mammals and can be reproduced in the laboratory. Ticks can also be directly infected with B. burgdorferi by several methods. Although genetic studies have only been undertaken in the past ten years, a number of genetic tools have been developed. Multiple selectable markers have been used for gene inactivation by allelic exchange. Shuttle vectors derived from endogenous and broad host-range plasmids have been constructed. A transposon mutagenesis system has been used to inactivate a number of genes. Limitations on genetic studies of B. burgdorferi remain. Transformation frequencies are very low, especially in infectious strains, in part because of plasmid-encoded restriction enzymes. Plasmids are unstable during in vitro growth, so care must be taken to ensure isogenicity of mutant and wild-type strains. Currently, the bacteria are only grown in complex medium, so many nutritional screens and selections are not possible. Despite these limitations, genes affecting a number of functions, including infection of mice or ticks, have been inactivated. Also, a gene that is essential for survival in all conditions has been identified.