Decolorization kinetics of a recombinant Escherichia coli strain harboring azo-dye-decolorizing determinants from Rhodococcus sp.

A 6.3 kb DNA fragment containing genes responsible for azo-dye decolorization was cloned and expressed in Escherichia coli. The resulting recombinant strain E. coli CY1 decolorized 200 mg azo dye (C.I. Reactive Red 22) l−1 at 28 °C at 8.2 mg g cell−1 h−1, while the host (E. coli DH5α) had no color-removal activity. Addition of 0.5 mM isopropyl-β-d-thiogalacto-pyranoside (IPTG) increased the decolorization rate 3.4-fold. The dependence of the decolorization rate on initial dye concentration essentially followed Monod-type kinetics and the maximal rate occurred with the dye at 600 mg l−1. The decolorization rate of E. coli CY1 was optimal at 40 °C and pH 11. Aeration (increased dissolved O2 level) strongly inhibited the decolorization, but decolorization occurred effectively under static incubation conditions (no agitation was employed). The CY1 strain also exhibited excellent stability during repeated-batch operations.