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) 1 ^-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 1 ^-1. The decolorization rate of E. coli CY1 was optimal at 40°C and pH 11. Aeration (increased dissolved O ₂ 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.