Reductive Decolorization of Azo Dye by Bacteria - Microbial Degradation of Synthetic Dyes in Wastewaters - page 122

ultimately concluded that bacterial azoreductase is indeed involved in resistance to
thiol-speci c stress caused by electrophilic quinones (Liu et al. 2009 ).

More and more evidences suggested that intracellular azoreductase might not
play a signi

cant role in bacterial decolorization of azo dye. Although bacterial
azoreductases could ef ciently decolorize azo dyes during in vitro tests, the

application of these enzymes for practical azo dye wastewater treatment seems to be
dif cult.

5 Mechanisms and Pathways of Bacterial Decolorization

The decolorization of azo dye is realized through the cleavage of azo bond. At
present, three mechanisms might be responsible for bacterial decolorization.

5.1 Direct Bacterial Decolorization

As mentioned in previous section, the participation of intracellular azoreductase in
bacterial decolorization still lacks evidence. However, the intense studies on azo
dye decolorization by Shewanella oneidensis MR-1 in recent years have provided
new insights of direct bacterial decolorization. The Mtr pathway of MR-1 is well-
known for the strain s excellent capabilities of extracellular electron transfer and

diverse respiration forms.
Brig

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et al. ( 2008 ) utilized random transposon mutagenesis and targeted inser-
tional mutagenesis to

é

nd important genes involved in bacterial decolorization of S.
oneidensis MR-1. It was found that the MTRA mutant was not completely de cient

in dye decolorization, suggesting that additional proteins are possibly involved in
the inter membrane electron transfer required for extracellular dye reduction.
Although only a small decrease in azo dye decolorization rate was observed with
omcA mutant, the reduction rates of omcA/omcB double mutants were signi

cantly
decreased compared with wild-type MR-1. The multi haem cytochromes MtrF,
OmcA, and OmcB may ful ll a function as terminal dye reductase. Furthermore,

azo dye decolorization studies have been carried out with MR-1 gene knock-out
mutant strains by Yu

s group in details recently (Cai et al. 2012 ; Cao et al. 2013 ). It
was found that the block of the Mtr pathway resulted in 80 % decrease of the
decolorization rate. Knockout of cymA resulted in a substantial loss of azo-reduc-
tion ability, which suggested that CymA is a key c-type cytochrome in the electron
transfer chain to azo dye. Therefore, MtrA-MtrB-MtrC respiratory pathway was
proposed to be mainly responsible for the anaerobic decolorization of azo dyes by
S. oneidensis MR-1. The potential involvement of membrane proteins of other
strains in azo dye decolorization remains yet to be studied.

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