Congo Red Decolorizing Bacteria from Paper Factory Effluent - Microbial Degradation of Synthetic Dyes in Wastewaters - page 138

was detected by immune electron microscopy in the vicinity of the cells which
suggests that the protein is not a typical extracellular enzyme, but presumably
released from the lysed cells (Russ et al. 2000 ). Most probably, anaerobic biological
azo dye reduction occurring outside the cells is catalyzed directly by periplasmic
enzymes or indirectly in a reaction with reduced electron carriers that are generated
by periplasmic enzymes (Kudlich et al. 1997 ).
However, Ra i et al. ( 1990 ) noted that the azo reductase enzyme was secreted

extracellularly in all of the bacteria isolated from the intestines as evidenced by the
presence of more enzymes in the culture supernatant. To further determine the exact
location of the dye degrading enzyme inside the cell, the azo reductase should be
puri

ed and characterized using SDS-PAGE and HPLC (Punj and John 2008 ).
A recent report described a high in vitro azo reductase activity of a cytoplasmic

avin reductase, a part of the ribonucleotide reductase complex in E. coli, but its
overexpression in Sphingomonas sp. strain failed to signi cantly increase the

in vivo reducing activity of the bacteria (Russ et al. 2000 ). An intracellular azo
reductase is responsible for decolorization activity as previously shown for other
azo dye degrading microorganisms (Roxon et al. 1967 ; Gingell and Walker 1971 ).
In general, azo reductase activity was only measured in the absence of oxygen
(Maier et al. 2004 ). It appears that the enzyme involved in azo dye degradation is
irreversibly inactivated in the presence of oxygen (Ra i et al. 1990 ).

The term “ anaerobic dye reduction ” comprises different mechanisms. A dis-
tinction can be made between direct enzymatic azo dye reduction and indirect azo
dye reduction catalyzed by enzymatically (re)generated redox mediating com-
pounds. It is also possible that azo dyes are chemically reduced by biogenic bulk
reductases like sulphide (van der Zee 2002 ).

6 Microbial Toxicity of Dyes and Their Degradation
Products

In our study, toxicity of decolorized culture medium was tested on prokaryotic cells
(Pseudomonas aeruginosa UPCC 1244 and Staphylococcus aureus UPCC 1143,
Gram negative and Gram positive bacteria, respectively) and eukaryotic cells
(Saccharomyces cerevisiae UPCC 2113, a yeast; Oryza sativa NSIC RC112, rice;
and Vigna radiata, mung bean). For microbial toxicity test, four treatments were
tested: decolorized medium on the 7th day of incubation, decolorized medium on
the 14th day of incubation, negative control and positive control (with Congo red).
For the plant toxicity test, the roots of selected rice and mung bean seedlings were
immersed overnight in the following treatments: decolorized medium on the 7th
day of incubation, decolorized medium on the 14th day of incubation, negative
control and 1 % Congo red solution (positive control). After incubation, the plants
were carefully uprooted from the media and the heights and dry weights were
determined. Heights were measured from the point separating the root and shoot to