Borrelia burgdorferi Linear Plasmid 28-3 Confers a Selective Advantage in an Experimental Mouse-Tick Infection Model

Abstract

Borrelia burgdorferi, the bacterium that causes Lyme disease, has a unique segmented genome consisting of numerous linear and circular plasmids and a linear chromosome. Many of these genetic elements have been found to encode factors critical for B. burgdorferi to complete the infectious cycle. However, several plasmids remain poorly characterized, and their roles during infection with B. burgdorferi have not been elucidated. To more fully characterize the role of one of the four 28-kb linear plasmids, lp28-3, we generated strains specifically lacking lp28-3 and assayed the contribution of genes carried by lp28-3 to B. burgdorferi infection. We found that lp28-3 does not carry any genes that are strictly required for infection of a mouse or tick and that lp28-3-deficient spirochetes are competent at causing a disseminated infection. Interestingly, spirochetes containing lp28-3 were at a selective advantage compared to lp28-3-deficient spirochetes when coinjected into a mouse, and this advantage was reflected in the population of spirochetes acquired by feeding ticks. Our data demonstrate that genes carried by lp28-3, although not essential, contribute to the fitness of B. burgdorferi during infection. Lyme disease is the most common tick-transmitted disease in the United States and is caused by infection with the pathogen Borrelia burgdorferi (1-3). This bacterium survives in a complex enzootic cycle transmitted by Ixodes ticks and infects a wide range of vertebrate hosts (4-8). B. burgdorferi must adapt to markedly different environments within the arthropod vector and mammalian host to ensure successful colonization, persistence, and transmission throughout the natural infectious cycle.The unique genome of the B. burgdorferi type strain B31 consists of a small 0.9-Mbp linear chromosome and a collection of 12 linear and 9 circular plasmids (9-12). Together, the plasmids comprise 0.533 Mbp and range in size from 5 kb to 56 kb (13-16). With the genetic tools now available to manipulate B. burgdorferi, a number of chromosomal and plasmid genes have been identified that encode factors required by this spirochete at particular stages of the infectious cycle.Plasmid-borne genes in B. burgdorferi have been shown to encode proteins required for viability, virulence, and fundamental metabolic processes (see reviews in references 17, 18, 19, 20, and 21). However, identification of such genes in B. burgdorferi by means of sequence homology alone has been difficult. Cumulatively, the strain B31 plasmids encode 706 putative proteins, 58% of which have no database match and 26% of which are conserved hypothetical proteins. In contrast, the B31 chromosome has 815 predicted open reading frames (ORFs), with only 29% lacking a database match and 12% annotated as conserved hypothetical proteins (14,22). Although a growing number of plasmid-borne genes have defined roles in the infectious cycle, most remain poorly characterized.Several B. burgdorferi plasmids have been shown to be required for or to contribute to survival in the ...

Apparent Role for Borrelia burgdorferi LuxS during Mammalian Infection

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