Physics of Bacteria Living in Gels: Host-Bacteria Interactions

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This proposal addresses the fundamental puzzle of how Helicobacter pylori (H. pylori) breaches the protective mucus barrier to reach the epithelial cell surface and colonize in the extreme acidic environment of the stomach. The studies proposed here will advance the fundamental physics underlying bacterial motion and aggregation in a viscoelastic gel, and elucidate the influence of shape on motility and colonization. Chemotactic measurements of bacteria moving in microfluidic devices with multiple chemical gradients across mucin/mucus layers on gastric cells will contribute to the development of a useful ex vivo, biomimetic model.Many bacteria, like those in the gastrointestinal tract, have to negotiate mucosal barriers to establish colonies. In particular, Helicobacter pylori, the bacterium that causes ulcers, gastritis and gastric cancer faces the additional challenge of overcoming the acidic environment. Previous work from the PI's group established that H. pylori uses the same biochemical strategy of urea hydrolysis to neutralize the acid and elevate the pH for both survival in the stomach and to swim across by triggering a gel-solution transition. Recent studies with mutants having cell shape alterations have shown that the helical shape is advantageous in establishing colonies in agar gels. Preliminary bacteria tracking microscopy and resistive force theory based modeling show that the distribution of swimming speeds is due to polydispersity of shape and size, along with a biphasic temporal variation of speed, and that the helical shape provides added propulsion in a viscous mucin solution leading to a faster swimming speed. Innovative imaging techniques will be used to investigate bacterial motility and the changes that the bacterium causes in the host gel by quantitative measurements of pH changes and alterations in local micro-rheology of its environment. The rheological studies of bacteria infected gels will be characterized using oscillatory shear rheology. Microfluidic geometries will be designed to mimic the gastric mucus environment under ex-vivo conditions, layering substrates with epithelial cells covered with mucus or purified mucin or gelatin or agar and establish pH or ionic gradients to examine the bacteria motility and the extent and pattern of colonization. The influence of the bacterium on the surrounding mucus /mucin will be probed by fluorescent dye binding and Atomic Force Microscopy. Comparative studies of the wild-type H. pylori with mutants with altered cell shapes or deficient in specific receptors or flagella, will make it possible to assess the effect of the many variables at play in this complex living system. The experimental studies will be complemented by computational and theoretical modeling of bacterial motility, body and flagella dynamics, chemotaxis, and pattern formation in gels to provide a quantitative understanding of the underlying physics. High school students will participate via the Summer research internship, RISE program at Boston University. Videos on bacterial motility and simple analysis tools will be developed for use in undergraduate courses and for demonstration purposes. The PI will take advantage of the diversity of the team, and the broad appeal of the research topic, to do outreach activities involving presentations to a broader audience, including local high schools, and minority and other community groups and to recruit women and minority graduate, undergraduate and high school students to participate in this research.