2.1 Degradation of azo dyes by azo reductases
Azo reductases catalyze the reaction only in presence of reducing equivalents like FADH and
NADH. Most of the azo dyes have sulphonate substituent groups and a high molecular
weight and they are unlikely to pass through cell membranes. Therefore, the reducing activity
referred to the dye is not dependant on the intracellular uptake of the dye (Robinson et al.,
2001). Russ et al. suggested that bacterial membranes are almost impermeable to flavincontaining
cofactors and, therefore, restrict the transfer of reduction equivalents by flavins
from the cytoplasm to the sulphonated azo dyes. Thus, a mechanism other than reduction by
reduced flavins formed by cytoplasmic flavin-dependent azoreductases must be responsible
for sulphonated azo dye reduction in bacterial cells with intact cell membranes (Russ et al.,
2000).
One such mechanism involves the electron transport-linked reduction of azo dyes in the
extra-cellular environment. To achieve this, the bacteria must establish a link between their
intracellular electron transport systems and the high molecular weight, azo dye molecules.
For such a link to be established, the electron transport components must be localized in the
outer membrane of the bacterial cells (in the case of gram-negative bacteria), where they can
make direct contact with either the azo dye substrate or a redox mediator at the cell surface
(Myers and Myers,1992). In addition, Gingell and Walker have shown that low molecular
weight redox mediator compounds can act as electron shuttles between the azo dye and an
NADH (nicotinamide adenine dinucleotide)-dependent azo reductase that is situated in the
outer membrane.