Sustainable Approach to the Degradation of Azo Dyes by Bacteria: A Review

Abstract

Azo dyes account for the majority of synthetic colorants discharged by the textile, leather, paper and food industries, and their recalcitrant azo bonds, together with the carcinogenic and mutagenic aromatic amines that can arise from their reductive cleavage, make them a persistent environmental and public health concern. Physicochemical treatments reduce colour but rarely achieve complete mineralisation and often generate secondary waste streams, which has sustained interest in bacterial bioremediation as a comparatively low-cost and potentially complete alternative. This review critically synthesises the literature on bacterial degradation of azo dyes, with particular attention to the enzymatic basis of decolorisation, the ecological and genomic organisation of degrading consortia, process engineering strategies that couple reductive and oxidative steps, the performance of extremophilic and genetically engineered strains under conditions representative of real effluents, and the analytical and computational tools now used to verify detoxification and optimise operation. The literature indicates broad agreement that azoreductase-mediated reductive cleavage under anaerobic or microaerophilic conditions is the obligatory first step in bacterial metabolism of most azo dyes, and that subsequent oxidative transformation of the resulting aromatic amines, whether by laccases, peroxidases or aerobic ring-cleavage pathways, is required for adequate detoxification. Evidence is more heterogeneous, and confidence correspondingly lower, regarding the generalisability of consortium performance across dye structures and salinities, the scalability of genetically modified strains beyond laboratory conditions, and the extent to which decolorisation percentages reported in isolated batch assays translate into demonstrable reductions in ecotoxicological endpoints. Methodological heterogeneity in dye concentration, inoculum history and toxicity assay selection substantially limits cross-study comparison. The review identifies specific priorities for future work, including standardised detoxification reporting, longer-term and mixed-dye continuous-flow trials, and closer integration of genomic characterisation with process design. Overall, bacterial systems offer a mechanistically well-understood and practically promising route to azo dye remediation, but their translation to industrial-scale, sustainable wastewater treatment remains constrained by inconsistent evidence on robustness, detoxification completeness and long-term operational stability.