The contribution of motility and chemotaxis in the Borrelia burgdorferi infectious life cycle

Abstract

Lyme disease has emerged as an increasing problem for people in the east and northeastern part of the United States. It can cause a chronic debilitating infection if left untreated and is difficult to diagnose. The illness is caused by an infection with the spirochete known as Borrelia burgdorferi. B. burgdorferi is a Gram-negative bacterium that is transmitted by ticks of the Ixodes genus. The primary carriers in regions of high Lyme disease incidence are Ixodes scapularis vector and white-footed Mus musculus rodents. B. burgdorferi is not known to produce common virulence factors such as toxins or capsules. Chemotaxis and motility are important for B. burgdorferi to cause infection and are considered as invasive attributes of this organism. Only a handful of studies have reported that non-chemotactic and non-motile B. burgdorferi mutants are unable to disseminate in hosts, and are, therefore, non-infectious in mice. Although motility and chemotaxis has been shown to be crucial for the infectious life cycle of B. burgdorferi, little is known about the mechanism of motility or assembly of periplasmic flagella. It is not known how incremental reductions of motility affect B. burgdorferi's virulence. Recent cryo-electron microscopy tomography revealed that spirochetes possess a unique flagellar structure called the collar. However, the gene or genes that encode for the B. burgdorferi collar proteins are unknown. Because of its location in the periplasmic flagellar motor, we hypothesize that this organelle is important for flagellar assembly as well as motility. We also hypothesize that less motile or less chemotactic mutants will exhibit a reduced invasive phenotype in vivo. Accordingly, using various comprehensive approaches, we have identified several putative genes in the B. burgdorferi genome with no significant similarity to other bacterial species. In vitro gene mutant analyses indicate that the cells are motility deficient. Additionally, fliZ putatively encodes a regulator of the flagella assembly complex. In other bacteria, inactivation of fliZ creates a reduced motile phenotype. To demonstrate if reduced motility is important for survivability or transmission between hosts, we plan to assay these mutants in mouse-tick-mouse infection cycle experiments. Importantly, mutants deficient in chemotaxis response regulator cheY2 exhibit normal motility and chemotaxis in vitro but exhibit reduced virulence in mice. Specifically, the cheY2 mutants were significantly attenuated in mouse infection and dissemination to distant tissues after needle inoculation. Additionally, while ΔcheY2 mutant cells can survive normally in the Ixodes ticks, mice fed upon by the ΔcheY2-infected ticks failed to establish persistent infection. These data suggest that CheY2, despite resembling a typical chemotaxis response regulator, functions distinctively from most other CheY proteins. These data lead us to propose that CheY2 serves as a regulator for a virulence determinant that is required for productive infection within murine, but not Ixodes tick hosts.