Adaptation of respiration and metabolism in Campylobacter jejuni

Summary

Campylobacter jejuni is the most frequent cause of human food-borne bacterial gastroenteritis worldwide, however also one of the least understood enteropathogens.Instead of being a pathogen for humans, C. jejuni is part of the commensal microbiota in many warm-blooded animals, especially poultry.The physiology of C. jejuni is distinct from model organisms and no regulatory proteins homologous to proteins from closely related organisms are present in its genome. This makes it impossible to predict how C. jejuni react to changes in its environment. C. jejuni harbors an extensive electron transport chain. Apart from oxygen, C. jejuni is able to use alternative electron acceptors like nitrate and fumarate for respiration. C. jejuni cannot utilize sugars for growth and instead feeds on amino- and organic acids. In order to survive and compete with the microbiota present in the intestine of animals, C. jejuni needs to adapt to changes in the environmental conditions. However, little is known how C. jejuni adapt to its environment. Experiments described in this thesis show that oxygen availability is a major determinant for adaptation of the electron transport chain of C. jejuni. When oxygen becomes limiting, alternative reductases are upregulated, as well as the formate dehydrogenase and hydrogenase enzymes, which ensures the generation of the membrane potential. Under oxygen limitation the two-component system RacRS plays a key role in fumarate metabolism. Fumarate can be used as carbon source or as electron acceptor at the cost of succinate excretion. The RacRS two-component system is activated when C. jejuni respires on alternative electron acceptors. Under these conditions, RacRS represses the use of fumarate as electron acceptor, resulting in decreased production of wasteful succinate. In vitro experiments show that recombinant RacR protein binds specifically to the promoter elements in front of the genes that are differentially regulated by RacR in vivo. Furthermore, the RacRS two-component system upregulates enzymes in glutamate synthesis pathways, increasing anabolism in favor of growth. Transcriptomic and functional analysis showed that the central metabolism of C. jejuni is regulated by carbon source availability. In the presence of the preferred carbon source serine the usage of other substrates is highly reduced. Growth on serine results in higher growth rate compared to growth on aspartate. Mutational and metabolomic analysis showed that an accumulation of intracellular succinate serves as cue for this metabolic reprogramming. This allows C. jejuni to distinguish between different nutrients, without the need for many specific regulatory proteins. Altogether, this thesis describes novel regulatory mechanisms that are used by C. jejuni to adapt towards changing environmental conditions that are likely happening in its natural habitat the intestine of warm-blooded animals. Because of the antibiotic resistance issues, these findings can serve as a valuable scientific basis for the development of new precise antimicrobial strategies.

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