Background: Compared with lactic acid-producing probiotics, spore-producing probiotics such as Bacillus subtilis (BS) and Bacillus natto (BN) exhibited superior metabolic capacity and stress resistance and are more suitable for industrial applications. However, limited understanding of their nutritional and intestinal health mechanisms has constrained their food potential. Objectives: This review systematically expounded on the ‘triple mechanism’ of BS and BN and their effects on intestinal nutrition, immunity and metabolism benefit for the first time. Methods: We searched PubMed, Scopus, Web of Science, and Google Scholar for studies on livestock, model organisms, and human research from 2000 to 2025. After evaluating relevance and eligibility, 115 articles were included. Results: Firstly, by secreting various digestive enzymes, BS and BN directly enhanced the small intestine digestive and absorptive efficiency and promoted animal growth. In particular, BN significantly increases calcium absorption in postmenopausal women. Secondly, as the antigen carrier that induced intestinal mucosal immunity, BS and BN enhanced the host’s defense ability by strengthening the expression of tight junction proteins, mucins, and inflammatory factors and bidirectionally regulated constipation and acute diarrhea in the human body. Thirdly, they reshaped the structure of the intestinal microbiota and their metabolic profile in the form of the gut–liver/gut–adipose axis, including enriching beneficial bacteria, activating lipid metabolism pathways such as PI3K/AKT and AMPK/SREBP, and regulating liver targets such as PPAR and CD36, thereby reducing insulin resistance and liver injury and maintaining overall metabolic homeostasis. Conclusions: Bacillus subtilis and Bacillus natto mediated their probiotic benefits through a gut-centric, multi-system regulatory strategy, involving nutrient utilization, immune homeostasis, and microbial–host metabolic interactions. This integrated mechanism provided a robust foundation for their targeted application in functional formulations and fermented food science.

This study investigated the mechanism underlying how the integration of chemical fertilizer and maize straw-derived organic amendments affects different potassium (K) pools, K saturation, K fixation characteristics, and non-exchangeable potassium (NEK) release kinetics in black soil. A field experiment involving five treatments was conducted: no fertilizer (CK), chemical fertilizer alone (NPK), chemical fertilizer plus maize straw (NPK+ST), chemical fertilizer plus straw compost (NPK+CP), and chemical fertilizer plus straw biochar (NPK+BR). The findings demonstrated remarkably increased contents of different K pools and K saturation in the black soil following the combined application of straw-derived organic amendments, with NPK+CP exhibiting the most significant effect. Exogenous K fixation simulation tests indicated that the amount of K fixation in the black soil increased with the incorporation of exogenous K, while the fixation rate showed the opposite trend. The quadratic regression model well fitted the relationship between the amount of K fixation and the addition of exogenous K; the treatments were ranked according to the soil K fixation capacity as follows: CK > NPK > NPK+BR > NPK+ST > NPK+CP. The simplified Elovich model well fitted the NEK release process in the black soil under different treatments. Significant differences in total cumulative NEK release and apparent release rate coefficients were observed among the treatments, which showed an identical trend: NPK+CP > NPK+BR > NPK+ST > NPK > CK. The application of straw-derived organic amendments could effectively reduce the K fixation capacity of black soil and promote NEK release. In this study, chemical fertilizer combined with straw compost was the optimal measure for improving black soil’s potassium availability.