Osmotic stress as a factor for regulating <i>E. coli</i> hydrogenase activity and enhancing H₂ production during mixed carbon sources fermentation

Abstract

<abstract> <p><italic>Escherichia coli</italic> performs mixed-acid fermentation and produces molecular hydrogen (H₂) via reversible hydrogenases (Hyd). H₂ producing activity was investigated during hyper- and hypo-osmotic stress conditions when a mixture of carbon sources (glucose and glycerol) was fermented at different pHs. Hyper-osmotic stress decreased H₂ production rate (V_H2) ~30 % in wild type at pH 7.5 when glucose was supplemented, while addition of formate stimulated V_H2 ~45% compared to hypo-stress conditions. Only in <italic>hyfG</italic> in formate assays was V_H2 inhibited ~25% compared to hypo-stress conditions. In hypo-stress conditions addition of glycerol increased V_H2 ~2 and 3 fold in <italic>hybC</italic> and <italic>hyfG</italic> mutants, respectively, compared to wild type. At pH 6.5 hyper-osmotic stress stimulated V_H2 ~2 fold in all strains except <italic>hyaB</italic> mutant when glucose was supplemented, while in formate assays significant stimulation (~3 fold) was determined in <italic>hybC</italic> mutant. At pH 5.5 hyper-osmotic stress inhibited V_H2 ~30% in wild type when glucose was supplemented, but in formate assays it was stimulated in all strains except <italic>hyfG</italic>. Taken together, it can be concluded that, depending on external pH and absence of Hyd enzymes in stationary-phase-grown osmotically stressed <italic>E. coli</italic> cells, H₂ production can be stimulated significantly which can be applied in developing H₂ production biotechnology.</p> </abstract>