INTRODUCTION

The Borrelia are long, highly motile, helical bacteria, 10— 30 μm long and 0.2—0.5 μm wide. They stain only weakly with Gram stain, and are usually visualized by dark-field microscopy.

Borreliae are unique among bacteria in that their genome is comprised of a linear chromosome and a variable number of both linear and circular plasmids. Despite the atypical form of the DNA, most genes encoded on their chromo­some are commonly found in other bacterial genomes. The genes encoded on the plasmid component of the genome, by contrast, appear to be unique to the genus Borrelia and show great variation between species and among strains¹-¹).

Classification, identification and phylogeny of B orrelia are complex. Before molecular biology, species designation and taxonomy were based on the morphology of the isolate, which vector the spirochaete was found in or which host animal the isolate was infective for. It is hence often impossible to elucidate to which ‘modern species’ older scientific reports refer.

Today, the genus Borrelia consists of at least 33 different species, of which 15—16 are currently regarded as part of the Lyme borreliosis spirochaetes group (LBS), whereas the remaining may be divided into a cluster of Old World relapsing fever spirochaetes (ORFS), a cluster of New World relapsing fever spirochaetes (NRFS) and a recently described monophyletic group of reptile-associated Bor- relia (Table 27.1). The LBS are transmitted principally by hard ticks in the Ixodes group, whereas the RFS are trans­mitted principally by soft body ticks in the genus Orni- thodoros. Future data may, however, challenge current concepts and divisions, and there seem to be at least one intermediate group of borrelial spirochaetes. Borrelia miya- motoi sensu lato, a species that genetically bears many similarities to NRFS, is for example isolated from both Ixodespersulcatus and I. ricinus in Asia and Europe⁽¹²⁾, and the closely related NRFS B. lonestari and B. theileri in North America are transmitted by ixodid ticks.