Helicobacter pylori requires flagellar motility and chemotaxis to establish and maintain chronic infection of the human stomach. The pH gradient in the stomach mucus is essential for bacterial orientation and guides the bacterium toward a narrow layer of the mucus, suggesting that H. pylori is capable of energy sensing or taxis. In the present study, H. pylori wild-type behavior in a temporal swimming assay could be altered by electron transport inhibitors, indicating that a connection between metabolism and behavior exists. In order to elucidate mechanisms of behavioral responses of H. pylori related to energy sensing, we investigated the phenotypes of single and multiple mutants of the four proposed chemotaxis sensor proteins. All sensor mutants were motile, but they diverged in their behavior in media supporting different energy yields. One proposed intracellular sensor, TlpD, was crucial for behavioral responses of H. pylori in defined media which did not permit growth and led to reduced bacterial energy levels. Suboptimal energetic conditions and inhibition of electron transport induced an increased frequency of stops and direction changes in the wild type but not in tlpD mutants. Loss of metabolism-dependent behavior in tlpD mutants could be reversed by complementation but not by electron donors bypassing the activity of the electron transport chain, in contrast to the case for the wild type. TlpD, which apparently lacks transmembrane domains, was detected both in the bacterial cytoplasm and at the bacterial periphery. The proposed energy sensor TlpD was found to mediate a repellent tactic response away from conditions of reduced electron transport.Helicobacter pylori has the ability to survive and multiply in the mucous layer of the human stomach, where it colonizes persistently for many years. Gastric Helicobacter species need complete flagella and full motility to colonize their hosts persistently (5,13,23,42). Both for the first entry of H. pylori into the human gastric mucus and for chronic colonization, H. pylori probably exploits horizontal and vertical substance gradients to reach and stay in its optimal habitat. Motile H. pylori mutants deficient in chemotaxis by knockout mutagenesis of the chemotaxis histidine kinase gene cheA and other chemotaxis proteins have been found to be unable to colonize in mouse and gerbil colonization models (16,26). Analysis of nanobiopsies taken from the stomach mucus of anesthetized, Helicobacterinfected mice and Mongolian gerbils has shown that Helicobacter actively accumulates in a very narrow zone of the proximal stomach mucus, at a distance from the epithelial surface of between 0 and 15 m (45, 46). It is guided there by a vertical proton gradient within the gastric mucus. Changes in other chemical gradients in the mucus in vivo did not alter the horizontal distribution of H. pylori, suggesting that pH taxis may be dominant over other chemotactic responses (45). pH taxis, a manifestation of energy-dependent taxis, was therefore shown to be essential for persiste...

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