The Borrelia burgdorferi RelA/SpoT Homolog and Stringent Response Regulate Survival in the Tick Vector and Global Gene Expression during Starvation

Abstract

As the Lyme disease bacterium Borrelia burgdorferi traverses its enzootic cycle, alternating between a tick vector and a vertebrate host, the spirochete must adapt and persist in the tick midgut under prolonged nutrient stress between blood meals. In this study, we examined the role of the stringent response in tick persistence and in regulation of gene expression during nutrient limitation. Nutritionally starving B. burgdorferi in vitro increased the levels of guanosine tetraphosphate (ppGpp) and guanosine pentaphosphate (pppGpp), collectively referred to as (p)ppGpp, products of the bifunctional synthetase/hydrolase Rel _Bbu (RelA/SpoT homolog). Conversely, returning B. burgdorferi to a nutrient-rich medium decreased (p)ppGpp levels. B. burgdorferi survival in ticks between the larval and nymph blood meals, and during starvation in vitro, was dependent on Rel _Bbu. Furthermore, normal morphological conversion from a flat-wave shape to a condensed round body (RB) form during starvation was dependent on Rel _Bbu; rel _Bbu mutants more frequently formed RBs, but their membranes were compromised. By differential RNA sequencing analyses, we found that Rel _Bbu regulates an extensive transcriptome, both dependent and independent of nutrient stress. The Rel _Bbu regulon includes the glp operon, which is important for glycerol utilization and persistence in the tick, virulence factors and the late phage operon of the 32-kb circular plasmid (cp32) family. In summary, our data suggest that Rel _Bbu globally modulates transcription in response to nutrient stress by increasing (p)ppGpp levels to facilitate B. burgdorferi persistence in the tick.

Author Summary

Borrelia burgdorferi, the spirochete responsible for causing Lyme disease, is maintained in nature via cycling between an Ixodes tick vector and a vertebrate host. The spirochete must adapt to and survive extreme nutrient deprivation, which may last months between blood meals, to persist in the midgut of the tick vector. How B. burgdorferi survives extended periods under such nutrient limitations has not been previously examined. In this study, we demonstrated that the stringent response, governed by Rel _Bbu, which synthesizes and hydrolyzes the alarmones guanosine tetraphosphate and guanosine pentaphosphate (collectively termed (p)ppGpp), is necessary for persistence in the tick. Rel _Bbu was also required for survival during in vitro starvation and rel _Bbu mutants more readily formed round bodies, a morphological change recently implicated in persistence in the tick. These adaptations to nutrient limitations appear to be mediated by global changes in gene expression modulated by Rel _Bbu activity. Our results highlight an important role for Rel _Bbu, and presumably (p)ppGpp, in vivo for persistence of a pathogen in its arthropod vector.