Competitive Advantage of Borrelia burgdorferi with Outer Surface Protein BBA03 during Tick-Mediated Infection of the Mammalian Host

Abstract

Linear plasmid lp54 is one of the most highly conserved and differentially expressed elements of the segmented genome of the Lyme disease spirochete Borrelia burgdorferi. We previously reported that deletion of a 4.1-kb region of lp54 (bba01 to bba07 [bba01-bba07]) led to a slight attenuation of tick-transmitted infection in mice following challenge with a large number of infected ticks. In the current study, we reduced the number of ticks in the challenge to more closely mimic the natural dose and found a profound defect in tick-transmitted infection of the bba01-bba07 mutant relative to wild-type B. burgdorferi. We next focused on deletion of bba03 as the most likely cause of this mutant phenotype, as previous studies have shown that expression of bba03 is increased by culture conditions that simulate tick feeding. Consistent with this hypothesis, we demonstrated increased expression of bba03 by spirochetes in fed relative to unfed ticks. We also observed that a bba03 deletion mutant, although fully competent by itself, did not efficiently infect mice when transmitted by ticks that were simultaneously coinfected with wild-type B. burgdorferi. These results suggest that BBA03 provides a competitive advantage to spirochetes carrying this protein during tick transmission to a mammalian host in the natural infectious cycle.T he genome of Borrelia burgdorferi, the causative agent of Lyme disease, consists of a linear chromosome and numerous linear and circular plasmids (9,15). Several of these extrachromosomal plasmids have been shown to carry genes that are critical for B. burgdorferi viability during the infectious cycle, as it must adapt to two very different environments, i.e., the Ixodes tick vector and a mammalian host (18,25,38,39,57). One of the most highly conserved plasmids, which also has one of the largest proportions of differentially regulated genes in the B. burgdorferi genome, is the linear plasmid lp54 (7,10,21,30,32,39,40,44,51,52,56). Although several genes on lp54 have been identified as contributing to (5,14,41,48) or essential for (3,39,58) spirochete fitness in vivo, the majority of the proteins encoded on this plasmid are of unknown function and lack homologs in other organisms (9,15). We previously used the Cre-lox system to delete the genes encompassing a 4.1-kb region of lp54, from bba01 to bba07 (bba01-bba07 region) (A3⌬A1-7 mutant) (4). In addition to a delay in colony formation and distinct colony morphology, a slight defect was observed in the ability of the A3⌬A1-7 mutant to establish mammalian infection by tick bite in a challenge with 20 infected nymphs per mouse. In this study, to create a more biologically relevant model, we reduced the tick challenge to 3 ticks per mouse and found that the A3⌬A1-7 mutant was significantly attenuated in the ability to infect mice by tick bite in this challenge, whereas wild-type (WT) B. burgdorferi could routinely establish mammalian infection at a dose as low as 1 tick per mouse.Since all attempts to complement the full A3⌬A1-7 mutant were unsuc...

Borrelia burgdorferi bba66 Gene Inactivation Results in Attenuated Mouse Infection by Tick Transmission

“…These data suggest that there was a greater probability of mice becoming infected with Bb ⌬A66 when more ticks fed to completion and may explain the results of the latter trials whereby mutant-infected mice usually had 4 or more ticks collected at the end of feeding. Bestor et al observed a similar phenomenon in experiments whereby an isolate lacking the genes bba01-07 was infectious by tick bite when 20 ticks per mouse were used, but not when the infestation was reduced to 3 ticks per mouse (37,38).…”

“…Recent research investigating borrelia-vector interactions have focused on inactivation of global regulators, individual genes, and replicons to advance our understanding of B. burgdorferi tick colonization and persistence and vector transmission mechanisms (1,4,7,18,(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50)(51)(52)(53)(54). The loss of BBA66 activity did not completely abolish the ability of B. burgdorferi to invade the host by tick bite (at least when more than 4 ticks/mouse fed to repletion), but mouse infectivity was significantly impaired.…”

Bacterial Heterogeneity Is a Requirement for Host Superinfection by the Lyme Disease Spirochete

“…The findings reported here demonstrate that homologous B. burgdorferi is unable to effectively coinfect or superinfect an immunocompetent mouse host. The inability of in vitro-grown B31-A3 clones to establish culture-detectable coinfection may pose a limitation to competitive infection experiments used to assess colonization defects of isogenic B. burgdorferi clones after coinoculation (57). The results also show that host adaptation of spirochetes does not enable homologous B. burgdorferi clones to superinfect C3H mice.…”

Comprehensive Spatial Analysis of the Borrelia burgdorferi Lipoproteome Reveals a Compartmentalization Bias toward the Bacterial Surface

“…The fourth protein in this set, BBA03, was also found solely in the IM in the present study. A prior well-controlled localization study remained equivocal in that BBA03 was found by immunofluorescence to be partially surface exposed but at the same time was protected from proteinase K in intact, but not permeabilized, cells (64). (ii) Two IM lipoproteins, BB0227 and BBB27, were localized to the inner leaflet of the OM in one of our earlier studies (57); we now believe that these localization results were erroneous due to contamination of the OMV fraction and a less-stringent interpretation of the data.…”