Isolation, Identification, and Genomic Characterization of a Cellulolytic Bacillus subtilis A2 from Goose Ileum

To identify efficient cellulose-degrading microbes suitable for the animal intestinal environment and to address the low utilization of crude fiber in feed, eight cellulolytic strains were isolated from the ileum of Yangzhou geese. Among them, strain A2 showed the highest cellulolytic activity (D/d = 1.48) via the CMC (carboxymethyl cellulose) agar transparent zone method. Based on whole-genome-based identification, strain A2 was identified as Bacillus subtilis. Whole-genome sequencing revealed a circular chromosome of 4.02 Mb with a GC content of 43.72%, containing 4083 protein-coding sequences, of which 7.40% were involved in carbohydrate transport and metabolism. CAZyme annotation identified 167 carbohydrate-active enzyme genes, including 64 glycoside hydrolase genes, along with 60 hemicellulase and 3 lignin-degrading enzyme genes, forming a complete lignocellulose-degrading system. The cellulase from A2 exhibited optimal activity at 55 °C and pH 7.0, with good stability at 50-65 °C and pH 5-7, and was significantly inhibited by Cu²⁺, Mn²⁺, and Zn²⁺. Notably, its degradation efficiency toward microcrystalline cellulose reached 197% of that toward CMC. In conclusion, B. subtilis A2, with its excellent enzymatic properties and robust genetic foundation, is a promising candidate for developing feed enzymes and enhancing lignocellulose utilization.