Biology, ecology and systematics of the genus borrelia

Biology, ecology and systematics of the genus borrelia

Zentralblatt fUr Zem.bL BakterioL 289, 639-642 (1999) © Urban & Fischer Verlag oumalsl zblbakterio I Extended Summary ...

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Zentralblatt fUr

Zem.bL BakterioL 289, 639-642 (1999) © Urban & Fischer Verlag oumalsl zblbakterio I

Extended Summary

Biology, Ecology and Systematics of the Genus Borrelia Klaus Kurtenbach, Henna-Sisko Sewell, Stefanie Magdalene Schafer, Susanne Etti, and Simona De Michelis The Wellcome Trust Centre for the Epidemiology of Infectious Disease, University of Oxford, United Kingdom NERC Institute of Virology and Environmental Microbiology, Oxford, United Kingdom

Introduction The genus Borrelia represents the medically most important group amongst tick-borne bacteria. This genus is divided into two major phylogenetic clusters, the relapsing fever spirochaetes and the B. burgdorferi sensu lato species­ complex. The latter comprises the causative agents of Lyme borreliosis. Spiro­ chaetes are known for their fastidious cultural requirements, a fact that has hindered phylogenetic, taxonomic and ecological studies for many years. Tra­ ditionally it is believed that the various species of the genus Borrelia co-evol­ ved with "their" arthropod vector. Therefore, many species, in particular many relapsing fever spirochaetes, have been named according to the vector species that they are transmitted by. However, the 'co-speciation' concept does not hold true for B. burgdorferi sensu lato, as most of the genospecies of this com­ plex are transmitted by Ixodes ricinus and/or I. persulcatus (Acari: Ixodidae). Key words: Borrelia, phylogeny, taxonomy, ecology

Systematics, biology and population structure Since the first description of B. burgdorferi in 1982, 10 genospecies have been delineated and named using Linnaean binomials. B. burgdorferi sensu stricto, B. afzelii and B. garinii are pathogenic to humans (10). The pathogenic po­ tential of the other genospecies (B. valaisiana, B. lusitaniae, B. turdi, B. tanu­ kii, B. japonica, B. andersonii, B. bissettii sp. nov.) is unclear. Molecular stud0934-8840/99/289/5-7-639 $12.00/0


K. Kurtenbach et al.

ies have revealed that B. burgdorferi s.l. has a linear chromosome, a rare fea­ ture in Eubacteria, and various linear and circular plasmids. Recently, the full genome of B. burgdorferi s. s. has been sequenced (2). Interestingly, approxi­ mately 50 % of the open reading frames, including those encoding the highly abundant outer surface proteins OspA and OspB, did not match any given data base entry. Therefore, based on sequence information alone, these genes of Borrelia cannot yet be assigned to a biological function. Studies on the pop­ ulation genetics of B. burgdorferi s.l. have revolutionised the understanding of the population structure and ecology of Lyme borreliosis. For example, it has been revealed that the population structure of B. burgdorferi s.l. is large­ ly clonal (1). The phylogenetic analysis of "clonal" genes or introns, such as ospA, fla or the rrf-rrl intergenic spacer, infers that the genospecies B. garinii and B. burgdorferi s. s. are evolutionarily the most ancestral (1, 10). Amongst the various genospecies B. garinii is the most heterogeneous and has further been structured into lower discrete typing units. Despite the overall clonality of B. burgdorferi s.l., molecular analyses of different genes from cultured strains indicate that horizontal transfer of gene­ tic material does occur within and between the genospecies. Such horizontal gene transfer seems to be confined to particular loci (4) as found in other path­ ogenic bacteria, and hence the term "localised sex" has been coined (11). The gene encoding the immunodominant OspC is one of the important recombin­ ing loci (4). The preliminary analysis of the population structure of ospC in local tick populations suggests that the genetic diversity is maintained by "bal­ ancing selection" (12). In fact, variable and immunodominant proteins, such as OspC or PG, are subject to immune selection pressure in the vertebrate host. However, current information on the genetic diversity of ospC of Bor­ relia circulating in natural tick or reservoir host populations is still poor. As OspC is a candidate for a therapeutic vaccine against Lyme borreliosis (13), precise knowledge of its genetic diversity is required in order to identify cross­ reactive epitopes.

Ecology In recent years it has become clear that the genospecies differ considerably phenotypically, in particular with regards to pathogenicity, serotype, transmis­ sibility, ecology and global distribution. B. burgdorferi s. s., and perhaps also B. bissettii sp. nov., have a transatlantic distribution, while all the other named genospecies (except for B. andersonii) are confined to the Old World. Amongst the Eurasian genospecies, B. garinii prevails from the Atlantic to the Pacific Ocean. In addition, transhemispheric exchange of B. garinii, due to pelagic seabird-tick transmission cycles, has been recorded (9). The large variety of different habitats in which B. garinii is circulating is reflected by a substantial genetic polymorphism of this genospecies. The geographic distri­ bution of the other Eurasian genospecies, i. e. B. afzelii, B. valaisiana, B. lusi-

Biology, ecology and systematics, of Borrelia burgdorferi s.l.


taniae, B. turdi, B. tanukii and B. japonica, seems to be more restricted than B. garinii, suggesting more specialised ecological niches. One of the most important ecological traits of B. burgdorferi s.1. is highly variable: host-to-tick transmissibility. For example, B. afzelii was found to be associated with rodent-tick transmission cycles (3), whereas particular geno­ types of B. garinii and B. valaisiana seem to be maintained by avian-tick trans­ mission cycles (3, 5). However, consistent with the observation that B. garinii is genetically diverse, not all genotypes of this genospecies seem to be associat­ ed exclusively with avian hosts. In Japan and Russia, for example, B. garinii­ related genotypes have been recorded that are transmitted to ticks by rodents (pers. communication: E. Korenberg, Moscow). Thus, distinct transmission cycles of B. burgdorferi s.1. exist involving a variety of host species, but only a few closely related tick species (e. g. I. ricinus and I. persulcatus) (5-8). There is evidence that transmissibility of the bacteria is linked to differential resistance to the alternative pathway of the complement system (6). It has been shown that complement from a variety of mammalian and avian species operates in a species-specific way independently of antibodies (6). In addition, data indicate that complement acts within the midgut of the tick during the uptake of blood (unpublished data). As B. burgdorferi s.1. displays antigenic phase variation in response to the microenvironment (e. g. up regulation of OspA in the midgut of the tick and downregulation in the vertebrate host), we hypothesise that pro­ teins specifically expressed in the tick mediate sensitivity or resistance to com­ plement. The differential lysis or survival of the bacteria within the midgut, de­ pending on their resistance to complement, renders the tick's midgut a "bottle neck" in terms of transmission of Borrelia. As a consequence, the analysis of resistance or sensitivity to complement in vitro could be exploited as a tool to predict putative reservoir-competent hosts of Borrelia strains whose ecology is unknown. Together with the plasminogen/plasmin system, complement can be regarded as an example of host derived molecules that act within the ectopara­ site. Acquisition of resistance to complement seems to be an important aspect in the adaptive radiation and evolution of B. burgdorferi s.1. Altogether, innate and adaptive immunity appear to play important roles in the evolution and population structure of Borrelia. Adaptive immunity (i. e. antibody and T cells) seems to impose selective constraints on variable proteins selectively expressed within the vertebrate host (e. g. OspC, pG), while innate, complement-mediated immunity appears to act on proteins expressed only in the tick's midgut (e. g. OspA, OspB), resulting in differen­ tial strain transmission.


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