Bacterial Degradation of Azo Dye Containing Wastes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 72

characterization of azo reductases reducing azo dyes were carried out by several
investigators. Aerobic FMN dependent azo reductases, reported for azo dye
decolorization, have been isolated from E. coli, Enterococcus faecalis, Kerstercia
sp. and Staphylococcus aureus. NADH dependent azo reductases have been
characterized in B. cereus, B. velezensis, B. badius and Bacillus sp. ADR. A FAD
independent azo reductase was isolated from Sphingomonas sp. by Kudlich et al.
( 1997 ). For better information up to sequence level, DNA screening and probe
design of NADPH dependent azo reductase (20 kDa) from Bacillus sp. OY1-2 was
studied by Suzuki et al. ( 2001 ). They also isolated gene from B. subtilis
ATCC6633, B. subtilis ISW1214 and G. stearothermophilus. Another 30 kDa
azo reductase enzyme was identi ed by Blumel et al. ( 2002 ) from Xenophilus

azovorans KF46F. Azo reductase from Pseudomonas aeruginosa was found to be
oxygen-insensitive towards azo dye degradation (Chen et al. 2005 ).

6 Mechanism of Bacterial Azo Dye Degradation

It is very much important to know the mechanism by which azo dye decolorization is
carried out by bacteria. Dye degradation studies are conducted both under aerobic as
well as anaerobic conditions. But generally bacterial degradation of azo dyes
comprises the reductive cleavage of azo bonds ( ) with the help of an azo

reductase enzyme under anaerobic conditions. During this process, four-electrons
(reducing equivalents) are transferred from electron donors to the electron acceptor
(azo dye) in two stages at the azo linkage, resulting in dye decolorization and
generation of colorless amines. The resulting intermediate metabolites (e.g., aro-
matic amines) are then further degraded aerobically or anaerobically. Azo dye
decolorization under anaerobic condition is simple but non-speci

N=N

- -

c process. Under

anaerobic conditions, a low redox potential ( 50 mV) causes the effective decol-

orization of the azo dyes. However, in case of aerobic degradation, respiration may
dominate utilization of NADH, thus inhibiting the electron transfer from NADH to
azo bonds. Alternatively, decolorization might be attributed to non-speci

≤

c extra-
cellular reactions occurring between reduced compounds generated by the anaerobic
biomass. Much of the experimental work involving the anaerobic decolorization of
dyes (predominantly azo dyes) was conducted using mono cultures. In anaerobic
conditions, the permeation of the azo dyes through cell membrane into the microbial
cells acts as the principal rate-limiting factor for the decolorization. Under aerobic
conditions, mono- and di-oxygenase enzymes catalyze the incorporation of oxygen
from O 2 into the aromatic ring of organic compounds prior to ring ssion. Some

aerobic bacteria are able to reduce azo compounds with the help of oxygen catalysed
azo reductases and produce aromatic amines. It was also reported that the aerobic azo
reductases were able to use both NAD(P)H and NADH as co-factors and reductively
cleaved not only the carboxylated growth substrates of the bacteria, but also the
sulfonated structural analogues. There are a few bacteria that are able to grow on
azo compounds as the sole carbon source. These bacteria cleave

N=N bonds

- -