Microbial Decolorization of Triphenylmethane Dyes - Microbial Degradation of Synthetic Dyes in Wastewaters - page 175

laccase (resulting in 3-methylbenzenesulponic acid formation) and subsequent des-
ulfonation by veratryl alcohol oxidase.

6 Microbial Toxicity of Dyes and Their Degradation
Products

The majority of studies concerning biodegradation of TPM dyes are focused on
mechanisms and factors involved in this process. In contrast, there are only a few
reports which include the evaluation of the toxicity of the obtained TPM dyes
metabolites. Such data are of great importance, because wastewaters decolorized by
biodegradation methods should be safe for the environment. For practical and
ethical reasons, mainly microorganisms, plants and invertebrates are used as model
organisms in toxicity assays. They are characterized by quick development and an
ability to produce resting forms. In microbial toxicity tests, the most often exploited
strains belong to bacterial species as K. rosea, P. aeruginosa, Azotobacter vine-
landii, S. paucimobilis, Escherichia coli and B. subtilis (Parshetti et al. 2006 ;
Shedbalkar et al. 2008 ; Ayed et al. 2009 ). In these tests, the toxicity of the original
dye and its by-products is usually expressed as a reduction in the number of
microbial cells per milliliter (with the use of liquid cultures) or as a formation of
inhibition zone surrounding the well lled with a dye solution (on solid media).

Chen et al. ( 2008 ) tested toxicity of Crystal violet and its possible degradation
products using the strain of E. coli. The parent dye occurred to be toxic, while the
Crystal violet intermediates formed by S. decolorationis NTOU1 were not toxic to
the test organism. While studying the biodegradation of Cotton blue by P.
ochrochloron MTCC 517, the bacterial toxicity test (with the use of A. vinelandii
strain) showed a growth inhibition zone (1.2 cm) around the well containing Cotton
blue, in contrast to the products of the dye transformation, which did not express
any inhibitory effect (Shedbalkar et al. 2008 ). Toxicity studies of Crystal violet and
its metabolites formed by a strain of A. radiobacter proved that degradation
products were less toxic to the strains of A. radiobacter, P. aeruginosa and A.
vinelandii (Parshetti et al. 2011 ).

7 Future Prospectives

Biodegradation of synthetic dyes using fungi and bacteria has become a promising
approach for the treatment of dye wastewaters. Now-a-days, molecular biology offers
various tools to accelerate natural abilities of organisms and optimize degradation
pathways. The principal approaches to develop genetically engineered microorgan-
isms, useful mainly in bioremediation include: modi

nity
of microbial enzymes and/or bioprocess monitoring and regulation. Moreover,

cation of speci city and af