Live imaging reveals a biphasic mode of dissemination of Borrelia burgdorferi within ticks

Abstract

Lyme disease is caused by transmission of the spirochete Borrelia burgdorferi from ticks to humans. Although much is known about B. burgdorferi replication, the routes and mechanisms by which it disseminates within the tick remain unclear. To better understand this process, we imaged live, infectious B. burgdorferi expressing a stably integrated, constitutively expressed GFP reporter. Using isolated tick midguts and salivary glands, we observed B. burgdorferi progress through the feeding tick via what we believe to be a novel, biphasic mode of dissemination. In the first phase, replicating spirochetes, positioned at varying depths throughout the midgut at the onset of feeding, formed networks of nonmotile organisms that advanced toward the basolateral surface of the epithelium while adhering to differentiating, hypertrophying, and detaching epithelial cells. In the second phase of dissemination, the nonmotile spirochetes transitioned into motile organisms that penetrated the basement membrane and entered the hemocoel, then migrated to and entered the salivary glands. We designated the first phase of dissemination "adherence-mediated migration" and provided evidence that it involves the inhibition of spirochete motility by one or more diffusible factors elaborated by the feeding tick midgut. Our studies, which we believe are the first to relate the transmission dynamics of spirochetes to the complex morphological and developmental changes that the midgut and salivary glands undergo during engorgement, challenge the conventional viewpoint that dissemination of Lyme disease-causing spirochetes within ticks is exclusively motility driven.

The heterogeneous motility of the Lyme disease spirochete in gelatin mimics dissemination through tissue

“…It is interesting to speculate as to why the motility states that we observe in the dermis and in gelatin may be important during the enzootic cycle. The tick midgut seems to be a hospitable environment for Bb, as the spirochetes are found in high abundance at this site (21,36,37). However, during the blood meal, some spirochetes must leave the tick and transition to the mammalian host for the species to survive (38).…”

“…Bb can bind directly to type I collagen (26) as well as other ECM constituents. To determine whether the motility of Bb in the dermis differs from that documented in liquid media and methylcellulose solutions, we used two-photon intravital confocal microscopy to track pathogen movement in the ears of mice that had been tick inoculated with Bb914, a virulent strain 297 isolate that constitutively expresses GFP (21). Spirochete movement was characterized by traveling-wave undulations of the cell body (Fig.…”

“…Gelatin, a denatured form of collagen, was chosen because it possesses many of the same binding sites as native collagen (25) and because it has been used as an in vitro equivalent to the ECM for the study of borrelial motility (21,26). We find that Bb in the dermis of chronically infected mice and in gelatin exhibit three or four motility states, which are determined by transient adhesion between the bacterium and the matrix.…”

“…To move from the tick to the mammal, Bb must migrate through many different tissues. In the tick, a small number of spirochetes exit the midgut during feeding by traversing a layer of epithelial cells and a thin, but dense, polymeric network known as a basement membrane (21). The spirochetes then swim through the hemocoel, a fluid environment containing hemocytes and hemolymph, where they attach to the salivary glands, penetrate another basement membrane, and enter the salivary ducts (22).…”

Lyme disease and relapsing feverBorreliaelongate through zones of peptidoglycan synthesis that mark division sites of daughter cells

“…The high passage variant of B31 MI, B31 clone e2, was generated and characterized previously (38). Bb914, a derivative of 297 that expresses cytoplasmic GFP from the stable episome cp26, has been described previously (39).…”