Abstract
Borrelia burgdorferi, the causative agent of Lyme disease in humans, is maintained in a complex biphasic life cycle, which alternates between tick and vertebrate hosts. To successfully survive and complete its enzootic cycle, B. burgdorferi adapts to diverse hosts by regulating genes required for survival in specific environments. Here we describe the first ever use of transposon insertion sequencing (Tn-seq) to identify genes required for B. burgdorferi survival in its tick host. We found that insertions into 46 genes resulted in a complete loss of recovery of mutants from larval Ixodes ticks. Insertions in an additional 56 genes resulted in a >90% decrease in fitness. The screen identified both previously known and new genes important for larval tick survival. Almost half of the genes required for survival in the tick encode proteins of unknown function, while a significant portion (over 20%) encode membrane-associated proteins or lipoproteins. We validated the results of the screen for five Tn mutants by performing individual competition assays using mutant and complemented strains. To better understand the role of one of these genes in tick survival, we conducted mechanistic studies of bb0017, a gene previously shown to be required for resistance against oxidative stress. In this study we show that BB0017 affects the regulation of key borrelial virulence determinants. The application of Tn-seq to in vivo screening of B. burgdorferi in its natural vector is a powerful tool that can be used to address many different aspects of the host pathogen interaction.
Highlights
Lyme disease is caused by the spirochete, Borrelia burgdorferi
The causative agent of Lyme disease, must adjust to environmental changes as it moves between its tick and vertebrate hosts
We performed a screen of a B. burgdorferi transposon library using massively parallel sequencing (Tn-seq) to identify fitness defects involved in survival in its tick host
Summary
Lyme disease is caused by the spirochete, Borrelia burgdorferi. In nature, B. burgdorferi is maintained in a cycle between mammalian or bird hosts and Ixodes ticks [1] Newly hatched ticks can acquire B. burgdorferi from infected animals during their larval feeding [1]. Previous studies have shown that B. burgdorferi adapts to its host environments through controlling the expression of proteins that aid in survival at specific points in its life cycle in its different hosts [7,8,9]. Proteins such as outer surface protein C (OspC), variablemajor- protein (Vmp)-like sequence E (VlsE) and decorin binding protein A (DbpA) are expressed to differing amounts during particular time points in the mammalian and tick phases of the B. burgdorferi life cycle [10,11,12,13,14]. The regulation of gene expression in B. burgdorferi is complex, often involving multiple layers of control [1,3,6]
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