Molecular Mechanisms of Bacterial Motility

Spirochetes are a phylogenetically distinct group of bacteria of significant importance to human health as they cause major diseases, such as syphilis (Treponema pallidum), Lyme disease (Borrelia burgdorferi), leptospirosis (Leptospira interrogans), and periodontitis (Treponema spp.). To infect and disseminate in mammalian hosts, spirochetes have evolved a unique morphology and motility that is highly effective at translocating through viscous media and tissue barriers. The organelles essential for spirochetal motility are periplasmic flagella, which reside in the bacterial periplasmic space and are distinct from external flagella in the model systems Escherichia coli and Salmonella enterica. Given that flagella-driven motility is crucial for virulence of pathogenic spirochetes and many other bacteria, our long-term goal is to reveal the molecular mechanisms underlying flagellar assembly and function. We demonstrated at unprecedented resolution that the Lyme disease spirochete B. burgdorferi (Bb) is a great model system for characterizing periplasmic flagella in situ. In collaboration with Drs. Md Motaleb and Chunhao Li, we generated and characterized a large Bb library, including 60 different flagellar and chemotaxis mutants. This work has significantly advanced understanding of the periplasmic flagella and their remarkable capacity in driving the unique spirochetal motility and morphology. We aim to illuminate three fundamental aspects of periplasmic flagella critical to virulence: 1) the structure and function of the flagellar type III secretion apparatus; 2) the mechanism underlying flagellar rotation driven by proton motive force across the membrane; and 3) the mechanisms by which flagella switch rotational directions to control motility and chemotaxis. Together with genetic and biochemical approaches, we use cryo-ET to determine the structure/function relationship of the spirochetal flagellar motor in its native cellular environment at nanometer resolution.