Chitinase biosynthesis and antifungal chitin bioconversion by Streptomyces mutabilis Ssg through statistical optimisation: an agro-industrial waste valorisation approach.

Chitin valorisation through enzymatic bioconversion offers a sustainable route for converting biowastes into value-added bioactive products. In this study, the chitinolytic strain Ssg, showing close phylogenetic affinity with Streptomyces mutabilis, was evaluated for chitinase production in a waste-integrated submerged fermentation system based on crustacean-derived colloidal chitin and supplemented with date seed and orange-peel powders as low-cost co-substrates. The isolate showed a high chitinolytic index of 17.20 ± 0.70. Plackett-Burman screening identified substrate composition and incubation time as key determinants of chitinase production, while Box-Behnken optimisation revealed significant interaction effects and identified the best-performing combination. At 10 g/L date seed powder, 4.08 g/L orange peel powder, and 72 h of incubation, chitinase activity reached 3196 ± 1.73 U/mL, closely matching the predicted value of 3198.01 U/mL. Optimisation increased enzyme production 7.6-fold and reduced fermentation time from 6 days to 72 h. Specific activity also increased from 4870 ± 170 to 13,620 ± 105 U/mg, reflecting stronger chitinolytic performance of the optimised preparation. Both the Optimised Cell-Free Supernatant and the crude chitin hydrolysate exhibited comparable antifungal activity against Aspergillus brasiliensis and Fusarium sp., with a slight advantage for the hydrolysate. The strongest response was observed against Fusarium sp. with the crude chitin hydrolysate, showing an inhibition zone of 24.00 ± 1.00 mm and mycelial growth inhibition of 38.75%. Microscopic observations revealed hyphal deformation and structural disorganisation. Overall, this study establishes a waste-integrated strategy for high-level chitinase production and supports the potential of the Ssg system for chitin bioconversion and biocontrol-oriented applications.