Degradation of azo dyes by Bacillus species: isolation and selection of new strains exhibiting enzymatic diversity with potential application in wastewater treatment

Dyes are widely used across various industries, and their incomplete fixation to fibers contaminates water. These compounds and their degradation products may be toxic, mutagenic, or carcinogenic, highlighting the need for efficient bioremediation strategies. This study reports the isolation of soil-derived Bacillus strains capable of decolorizing multiple azo dyes within a few days. Three strains, designated C7.1, L6, and L7, exhibited the highest decolorization rates under static conditions. Whole-genome analysis indicated that strains C7.1 and L7 are affiliated with the Bacillus cereus sensu lato group, whereas L6 is closely related to Bacillus velezensis. Genes encoding azoreductases, key enzymes responsible for the reductive cleavage of the azo chromophore, were identified in all strains, suggesting their central role in dye degradation. Additionally, B. velezensis (strain L6) has sequences encoding spore coat CotA laccase and DyP-type peroxidase, enzymes implicated in the biodegradation of other xenobiotic compounds. Cytotoxicity assays using Vero cells demonstrated that, after seven days of cultivation in media containing azo dye Methyl Orange, cell viability increased after treatment with strains C7.1 and L6. These findings suggest that the selected Bacillus isolates are promising candidates for bioremediation of dye-contaminated effluents and warrant further investigation to elucidate the metabolites and enzymatic pathways involved.