Homologous overexpression of azoreductase (azoA) in Enterococcus sp. L2 moderated growth and azo dye decolorization while gaining an oxidative and heavy metal stress resistance: A trade-off

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Heavy metals as co-pollutants significantly restrict the efficiency of bioremediation of azo dye-containing effluents wherein bacterial survival is a prerequisite. An exposure to quinones and oxidative stress via H 2 O 2 could induce the azoreductase activity in Enterococcus sp. L2 which could link the AzoA to its stress management system. Therefore, we underpinned the physiological and stress tolerance features by homologous over-expression of the azoA gene in its native strain. AzoA overexpression increased 3.4 folds azoreductase specific activity and enhanced quinone reduction profile. AzoA overexpression in native strain led to a delay in the early log phase growth and reactive violet 5R decolorization while gaining an advantageous stress resistance towards oxidative and heavy metals such as copper and chromium. This was confirmed by the 32 folds higher survival upon exposure to 20 mM H 2 O 2 supported by H 2 DCFA staining. Additionally, the genetically modified strain significantly tolerated 3 mM Cu(II) and 0.3 mM Cr(VI) supporting the development of a robust azo dye bioremediation strain. • Azo dye, quinones and H 2 O 2 enhanced azoreductase activity of Enterococcus sp. L2. • Homologous cloning in strain L2 resulted in 3.4 folds over-expression of AzoA gene. • AzoA over-expression in strain L2 moderated growth and dye decolorization. • AzoA over-expression in strain L2 increased oxidative stress (H 2 O 2 ) resistance. • AzoA over-expression in strain L2 enhanced heavy metal (Cu 2+ and Cr 6+ ) tolerance.