Hydrogen use by bacterial enteric pathogens
Zbell, Andrea Lynn
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Uptake-type hydrogenases catalyze the oxidation of molecular hydrogen in an energy-conserving manner. Many prokaryotes use these hydrogenases in their central metabolism. It has been recently shown that pathogens can use H2 as an energy source during host colonization. Salmonella enterica serovar Typhimurium contains three uptake-type hydrogenases: Hya, Hyb and Hyd. Since all of these hydrogenases can catalyze the oxidation of H2, it was hypothesized that they are used under different conditions and are differentially regulated. Studies with hydrogenase promoter fusions to lacZ revealed that hya is preferentially expressed during fermentative growth. Hya gene expression was activated by FNR and repressed by NarL. It may be involved in recycling anaerobically- produced H2, since hydrogen evolution was not observed in a double mutant that only contained Hya. Studies using resolvase in vivo expression technology showed that the bacterial hya genes were also expressed in S. enterica serovar Typhimurium cells residing within macrophages and during mouse infection. Hya seems to be very important for pathogenesis, since it was necessary for acid-resistance and colonization of macrophages. Interestingly, the hyd genes were upregulated during aerobic growth and were repressed by ArcA. The hyd genes were highly expressed in bacteria residing within macrophages and in the mouse liver and spleen. The hyb genes were upregulated during anaerobic respiration, and may be expressed in bacteria residing within macrophages. Hyb was repressed by IscR and in the presence of glucose, and was activated by ArcA and FNR. Growth studies indicated that the presence of hydrogen augmented growth during anaerobic respiration, and Hyb is an important hydrogenase for this phenomenon. Collectively, the in vivo and in vitro results demonstrate that each hydrogenase is differentially expressed and suggest that each has a unique role in the physiology of S. enterica serovar Typhimurium. Shigella flexneri is another important enteric pathogen that contains two uptake-type hydrogenases, Hya and Hyb. Whole-cell hydrogenase assays showed that hydrogenase activity was higher when cells were grown via anaerobic respiration with fumarate, as compared to cells grown anaerobically on blood-agar. Activity was highest in anaerobic or microaerobic cultures, as compared to cells grown under atmospheric conditions. Studies with hydrogenase mutants indicated that Hya is important when cells are grown anaerobically on rich media, while Hyb is important in cells grown under anaerobic respiration conditions.