Summary Tetrathionate (S4O6 2-) is used by some bacteria as an electron acceptor and can be produced in the vertebrate intestinal mucosa from the oxidation of thiosulphate (S2O3 2-) by reactive oxygen species during inflammation. Surprisingly, growth of the microaerophilic mucosal pathogen Campylobacter jejuni under oxygen-limited conditions was stimulated by tetrathionate, although it does not possess any known type of tetrathionate reductase. Here, we identify a dihaem cytochrome c (C8j_0815; TsdA) as the enzyme responsible. Kinetic studies with purified recombinant C. jejuni TsdA showed it to be a bifunctional tetrathionate reductase/thiosulphate dehydrogenase with a high affinity for tetrathionate. A tsdA null mutant still slowly reduced, but could not grow on, tetrathionate under oxygen limitation, lacked thiosulphate-dependent respiration and failed to convert thiosulphate to tetrathionate microaerobically. A TsdA paralogue (C8j_0040), lacking the unusual His-Cys haem ligation of TsdA, had low thiosulphate dehydrogenase and tetrathionate reductase activities. Our data highlight a hitherto unrecognized capacity of C. jejuni to use tetrathionate and thiosulphate in its energy metabolism, which may promote growth in the host. Moreover, as TsdA represents a new class of tetrathionate reductase that is widely distributed among bacteria, we predict that energy conserving tetrathionate respiration is far more common than currently appreciated.

The growing consumer demand for low-calorie, sugar-free foodstuff motivated us to search for alternative non-nutritive sweeteners. A promising sweet-tasting compound is 5-keto-D-fructose (5-KF), which is formed by membrane-bound fructose dehydrogenases (Fdh) in some Gluconobacter strains. The plasmid-based expression of the fdh genes in Gluconobacter (G.) oxydans resulted in a much higher Fdh activity in comparison to the native host G. japonicus. Growth experiments with G. oxydans fdh in fructose-containing media indicated that 5-KF was rapidly formed with a conversion efficiency of 90%. 5-KF production from fructose was also observed using resting cells with a yield of about 100%. In addition, a new approach was tested for the production of the sweetener 5-KF by using sucrose as a substrate. To this end, a two-strain system composed of the fdh-expressing strain and a G. oxydans strain that produced the sucrose hydrolyzing SacC was developed. The strains were co-cultured in sucrose medium and converted 92.5% of the available fructose units into 5-KF. The glucose moiety of sucrose was converted to 2-ketogluconate and acetate. With regard to the development of a sustainable and resource-saving process for the production of 5-KF, sugar beet extract was used as substrate for the two-strain system. Fructose as product from sucrose cleavage was mainly oxidized to 5-KF which was detected in a concentration of over 200 mM at the end of the fermentation process. In summary, the two-strain system was able to convert fructose units of sugar beet extract to 5-KF with an efficiency of 82 ± 5%.

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The gut microbe Akkermansia muciniphila is important for the human health as the occurrence of the organism is inversely correlated with different metabolic disorders. The metabolism of the organism includes the degradation of intestinal mucins. Thus, the gut health‐promoting properties are not immediately obvious and mechanisms of bacteria‐host interactions are mostly unclear. In this study, we characterized a novel extracellular β‐galactosidase (Amuc_1686) with a preference for linkages from the type Galβ1–3GalNAc. Additionally, Amuc_1686 possesses a discoidin‐like domain, which enables the interaction with anionic phospholipids. We detected a strong inhibition by phosphatidylserine, phosphatidylglycerol, phosphatidic acid, and lysophosphatidic acid while phosphatidylcholine and phosphatidylethanolamine had no influence. Amuc_1686 is the first example of a prokaryotic hydrolase that is strongly inhibited by certain phospholipids. These inhibiting phospholipids have important signal functions in immune response and cell clearance processes. Hence, Amuc_1686 might be regulated based on the health status of the large intestine and could therefore contribute to the mutualistic relationship between the microbe and the host on a molecular level. In this sense, Amuc_1686 could act as an altruistic enzyme that does not attack the mucin layer of apoptotic epithelial cells to ensure tissue regeneration, for example, in areas with inflammatory damages.

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