Our proposed research focuses on defining the mechanism of action by which the Helicobacter pylori TlpD chemoreceptor senses reactive oxygen species (ROS) and contributes to infection. TlpD is the most critical H. pylori chemoreceptor for gastric colonization. There is a fundamental gap, however in our understanding of how TlpD or its relatives detect oxygen radicals and, in turn, allow H. pylori to thrive in the gastric environment, a gap that we will fill by experiments proposed here. Continued existence of this gap prevents us from gaining a full understanding of H. pylori's pathogenic mechanisms and, in the long term, thwarting these processes to enable the creation of new drugs against H. pylori-related disease. Millions of people worldwide and in the U.S. are infected by H. pylori and suffer from its associated diseases-ulcers and gastric cancer. Gastric cancer is the second cause of cancer deaths worldwide. H. pylori is here to stay, however, based on recent studies that show H. pylori incidence has stabilized in the developed world. Furthermore, current therapies to cure H. pylori infection fail with unacceptable frequency, e.g., recent estimates in the United States have found that 20-25% of infected individuals are not cured by the current therapeutic regime. The overall objective of this application is to determine the molecular mechanisms by which TlpD detects ROS and promotes H. pylori's ability to thrive in the stomach. Our central hypothesis is that TlpD senses ROS directly, through amino acid oxidation, and drives a chemotactic repellent response that protects H. pylori from hydrogen peroxide exposure in the stomach. Our hypothesis has been formulated from preliminary data that determined that TlpD directly drives a repellent chemotaxis response to ROS and is critical for gastric colonization. In Aim 1, we use mass spectrometry to identify which specific TlpD amino acids are oxidized and determine the role of these amino acids in the ROS response. We will also determine the diversity of oxidants that can activate TlpD, and will test specific models for TlpD function. In Aim 2, we will assess whether H. pylori is exposed to hydrogen peroxide in vivo from the hydrogen peroxide generating DUOX2 oxidase, using transgenic mice, whether TlpD mitigates this exposure, and how ROS compares between stomach regions where TlpD is needed to greater and lesser degrees. The proposed research is innovative in that it covers novel hypotheses for both the basic mechanism of ROS chemotaxis and its role in infection. The proposed research is significant because it will provide new basic understanding of a novel way to avoid ROS exposure, ascertain the mechanism by which a widespread set of unstudied chemoreceptors senses ROS, provide improved understanding of bacterial processes that underlie colonization by a significant pathogen, and enhanced knowledge of the process by which chemotaxis promotes colonization. The long-term outcomes generated by this research are likely to provide insights that will enable creation of new drugs against H. pylori-related disease, including those that that inhibit the TlpD system and, thus, drive bacterial pathogens toward extensive ROS exposure and death.