Enhanced biofilm-mediated degradation of carcinogenic and mutagenic azo dye by novel bacteria isolated from tannery wastewater

Bacterial biofilm and extracellular polymeric substances (EPS) has emerged as an attractive solution for removing dyes from wastewater. Objectives of this study were to identify the biofilm forming bacteria (BFB), contents of the biofilm-EPS, and quantify the effect of novel BFB on biodegradation of Congo red from synthetic wastewater. In this study, 53.3% (16 out of 30) bacterial strains built fragile – robust biofilms in the glass test tubes. Biochemical (Congo red- and Calcofluor binding), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy results revealed biofilm-EPS consist of curli fimbriae, cellulose, proteins, lipids, nucleic acids and peptidoglycans. Bacterial strains those decolorized > 90% of azo dyes were sequenced. Four novel azo dye-decolorizing biofilm forming bacteria including Bacillus circulans ES6T, Staphylococcus cohnii ES9T, Proteus pennerii ES17T, ES24T, ES25T and ES31T, and Klebsiella quasipneumonae ES22T were identified. Color reduction of Congo red (CR) by ES6T, ES9T, ES17T and ES22T was optimized using the single factor batch experiments. These bacterial strains under optimum conditions (i.e., salt-optimized broth containing 2% sucrose, 80 mg L−1 CR, 1.5% NaCl, at pH 8, 35 ºC, under microaerophilic condition within 72 h of incubation) removed 96.8 – 99.4% CR. Synthesis of total azoreductase (39.6–43.4-fold) and laccase (22.5–33.2-fold) enzymes was induced under optimal conditions. The CR degradation was validated using FTIR and UV-Vis spectral studies. This is the first report on decolorization and degradation of CR by biofilm-EPS producing bacterial strains of B. circulans ES6T, S. cohnii ES9T, P. pennerii ES17T and K. quasipneumonae ES22T.