Helicobacter pylori is a human gastric pathogen that colonizes half the world's population and causes a wide range of diseases including gastritis, ulcers, and cancer. The distribution of the bacteria in the stomach is correlated with disease outcomes, but the factors that determine this distribution are unknown. We hypothesize that chemotaxis, the ability of bacteria to sense and orient their movement in response to chemical gradients in their environment is a key determinant of H. pylori distribution in the stomach. H. pylori is ideally suited for testing the role of bacterial chemotaxis in tissue distribution, becaue it has a simple chemotaxis system with only three core chemoreceptors and it colonizes an organ with extreme chemical gradients. This proposal builds on our novel insights into the ligands and sensing mechanisms of H. pylori chemoreceptors. In this proposal, we will use structural, biophysical, and biochemical approaches to understand the precise mechanisms by which H. pylori chemoreceptors sense three key chemicals that define three important axes in the stomach environment: urea (secreted from the gastric epithelium, defining the epithelial to luminal axis), the bacterial-produced quorum sensing molecule autoinducer-2 (defining regions of high bacterial density in the gastric glands versus the overlying mucosa), and acid (produced by the parietal cells in the anterior corpus region, defining the corpus to antrum, as well as the epithelial to luminal axis in the corpus). We have engineered mutant H. pylori that are specifically defective in sensing one of these three chemicals and we have developed highly sensitive assays for quantifying H. pylori chemotactic responses. We will use these existing mutants, and others that we engineer based on our new insights into the chemosensing mechanisms, to understand how H. pylori cells organize themselves in response to chemical signals on cultured epithelial monolayers and during infection of the mouse stomach. Knowledge gained from these studies will lead to new strategies for preventing and treating H. pylori infections by altering the bacteria's distribution in the stomach. More generally, these studies will provide novel insights into how bacterial pathogens perceive and navigate the host environment, knowledge which will be crucial for designing the next generation of antibiotics that specifically target within-host properties of bacterial invaders.