Characterization of azo dye reduction in Enterococcus faecalis

Abstract

Scope and Method of Study:

Majority of the dyes used in the paper, textile, food and pharmaceutical industries are azo dyes. Intestinal bacteria play an important role in the reduction of these dyes. Azoreductase, produced by intestinal microbiota, cleave the azo bond (N=N) in these dyes to produce colorless compounds, some of which are carcinogenic. Enterococcus faecalis , an intestinal bacterium is known to reduce a variety of azo dyes. The current study focuses on the physiological effects of three azo dyes, Methyl Red, Tartrazine and Direct Blue 15 on E. faecalis as well as the biochemical purification and characterization of a native azoreductase from this organism. Domain homology was also used as a criterion to identify and classify azoreductases.

Findings and Conclusions:

The results indicate that azo dyes are reduced at the highest rate under anaerobic conditions in actively dividing cells. Majority of the water-soluble azo dyes, Tartrazine and Direct Blue 15 were reduced externally but Methyl Red was reduced to an equal extent in the cytoplasmic fraction as well. NAD(P)H can serve as an electron donor for dye reduction. Most of the azoreductases contain either an FMN_red or flavodoxin_2 domain as defined by the Pfam database. These domains can serve as an additional criterion for identifying and classifying azoreductases. The enzyme activity of the native azoreductase (AzoA) from E. faecalis is a 100-fold more active than the heterologously expressed enzyme in E. coli and hence data from heterologously expressed enzymes must be interpreted with caution. AzoA can utilize NAD(P)H as electron donors for Methyl Red reduction, although it has a greater affinity for NADH over NADPH.