Energy Taxis Drives Campylobacter jejuni toward the Most Favorable Conditions for Growth

Campylobacter jejuni is a serious food-borne bacterial pathogen in the developed world. Poultry is a major reservoir, and C. jejuni appears highly adapted to the gastrointestinal tract of birds. Several factors are important for chicken colonization and virulence, including a taxis mechanism for environmental navigation. To explore the mechanism of chemotaxis in C. jejuni, we constructed mutants with deletions of five putative mcp (methyl-accepting chemotaxis protein) genes (tlp1, tlp2, tlp3, docB, and docC). Surprisingly, the deletions did not affect the chemotactic behavior of the mutants compared to that of the parental strain. However, the tlp1, tlp3, docB, and docC mutant strains displayed a 10-fold decrease in the ability to invade human epithelial and chicken embryo cells, hence demonstrating that the corresponding proteins affect the host interaction. l-Asparagine, formate, d-lactate, and chicken mucus were identified as new attractants of C. jejuni, and we observed that chemical substances promoting tactic attraction are all known to support the growth of this organism. The attractants could be categorized as carbon sources and electron donors and acceptors, and we furthermore observed a correlation between an attractant''s potency and its efficiency as an energy source. The tactic attraction was inhibited by the respiratory inhibitors HQNO (2-n-heptyl-4-hydroxyquinoline N-oxide) and sodium azide, which significantly reduce energy production by oxidative phosphorylation. These findings strongly indicate that energy taxis is the primary force in environmental navigation by C. jejuni and that this mechanism drives the organism toward the optimal chemical conditions for energy generation and colonization.The food-borne pathogen Campylobacter jejuni is highly adapted to the environment of the avian gut, where the mucus-filled crypts of the lower gastrointestinal tract are the primary site of colonization (6). It has been speculated that C. jejuni bacteria apply chemotaxis to reach this particular milieu (10, 42). Chemotaxis allows motile bacteria to navigate according to the extracellular chemical composition. The bacteria are either attracted or repelled by chemicals sensed by trans-membrane methyl-accepting chemotaxis proteins (MCP), and the information is transmitted to the flagellum motor via the histidine kinase CheA and the response regulator CheY. In contrast to the classical, metabolism-independent chemotaxis, some bacteria, such as Azospirillum brasilense and Rhodobacter sphaeroides, display metabolism-dependent “energy taxis” (or redox taxis) in which the signal for navigation originates within the electron transport system (1, 37).C. jejuni NCTC11168 encodes 10 MCP-like proteins, termed Tlp (transducer-like proteins), and two proteins with homology to aerotaxis receptors (Aer) (31). A taxis mechanism is essential for C. jejuni colonization, since strains with mutations of the central histidine kinase, cheA, or the response regulator, cheY, are unable to establish colonization in mice, chickens, and ferrets (10, 20, 51). Furthermore, C. jejuni strains with mutations in docB (tlp10) and docC (tlp4) are severely impaired in establishing colonization in chickens, whereas none of the other putative receptors are required for colonization (however, tlp5 could not be mutated) (20). Mutants of aer2 (cetB) and tlp9 (cetA) displayed reduced migration in minimal medium supplemented with pyruvate or fumarate, which leads to the speculation that CetA and CetB mediate an energy taxis response of C. jejuni (19).C. jejuni is attracted to amino acids, organic acids, or mucus components, while it is repelled by bile components (23). However, specific Tlp proteins have not been matched to any of these substances. It is speculated that the attraction toward chicken mucus directs and retains C. jejuni in the optimal environment of the avian intestinal lumen and thus prevents direct interaction with epithelial cells. This notion is based on in vitro observations where chicken mucus inhibited C. jejuni invasion of primary human epithelial cells, while increased invasion was observed for mutants carrying deletions of either cheA or cheY (9, 44, 51).To explore the mechanism of C. jejuni chemotaxis and analyze the biological functions of individual MCP-like proteins, we have analyzed five mutants with deletions of tlp genes (tlp1, tlp2, tlp3, docB, and docC). Furthermore, we have explored whether C. jejuni is primarily driven by chemotaxis or energy taxis.