Hydrolysis of palm kernel meal fibre using a newly isolated Bacillus subtilis F6 with high mannanase activity

Palm kernel meal (PKM) presents a challenge for non-ruminant livestock feeding due to its high fibre content predominantly in the form of mannan. Microbial fermentation offers a sustainable solution for fibre hydrolysis in lignocellulosic biomass. In this study, a Bacillus subtilis strain (F6), with high mannanase secretion capability, was isolated from the environment. Fermentation of PKM with B. subtilis F6 resulted in at least a 10% reduction in neutral detergent fibre, decreasing from 78.4 to 60.9% within 24 h. Notably, B. subtilis F6 rapidly responded to PKM, producing significant mannanase activity within 6 h, facilitating quick fibre degradation. Transcriptome analysis identified key enzymes involved in this process, with β-mannanase GmuG showing the highest increase in expression (45.2-fold) after fermentation. Purified recombinant GmuG exhibited strong PKM hydrolysis activity, primarily releasing mannobiose and mannotriose. Characterization of GmuG using locust bean gum as a substrate revealed an optimum temperature of 50-55 °C and pH optima at 5.0 and 9.0. This study highlights the potential of B. subtilis F6 and its mannanase GmuG for efficient PKM fibre hydrolysis, and provides insights into their application in the valorization of mannan-rich bioresources.