Cellulase production by Bacillus smithii QT03 using agro-industrial waste as carbon source

• First report of cellulase production by B. smithii using agro-industrial residues. • Optimized conditions increased cellulase yield using malt bagasse carbon source. • Cellulase was most active at 70 °C and 70 % stable at pH 6 for 24 h. • Enzyme retained 100 % activity at 55 °C for 3 h and 58 % at 60 °C for 24 h. • B. smithii QT03 is promising for sustainable enzyme production in biotechnology. Currently, within the concepts of Bioeconomy and Circular Economy, technologies for transforming agro-indutrial waste into high-value products are rapidly advancing and gaining attention. Among these, microbial enzymes like cellulase have become valuable due to their extensive industrial application and potential to reduce cultivation cost, especially by using agro-industrial residues as substrates. This study aimed to optimize the culture conditions of Bacillus smithii QT03 for cellulase production and to partially characterize the enzyme. Several parameters were evaluated, including different agro-industrial wastes as a carbon source, incubation temperature, pH, incubation time, agitation, inoculum size, nitrogen source, and cellulase thermal and pH stability. B. smithii QT03 produces cellulase in the presence of malt bagasse as a carbon source (2 %), meat extract as a nitrogen source (1.5 %), at an incubation time of 48 h at 35 °C, stirring at 200 rpm, and 2 % inoculum. The enzyme reached maximum activity after 30 min at pH 7 and maintained 70 % of its activity for 24 h at pH 6. It also reached maximum activity at 70 °C and maintained 100 % activity after incubation at 55 °C for 3 h, and 58 % at 60 °C for 24 h. These results suggest that cellulase production by B. smithii QT03 using malt bagasse presents a promising, sustainable alternative for industrial applications.