Combined Supplementation with Streptococcus Thermophilus Yilife-SST 01 and Bifidobacterium Bifidum BL-99 Mitigates Unloading-Induced Osteoporosis in Rats.

Probiotics have emerged as promising modulators of bone metabolism. This study evaluated the bone-protective effects of Streptococcus thermophilus Yilife-SST01 alone or in combination with Bifidobacterium bifidum BL-99 using MC3T3-E1 osteoblasts and a hindlimb unloading-induced osteoporosis model in Sprague-Dawley rats. S. thermophilus Yilife-SST01 promoted osteoblast mineralization and significantly inhibited osteoclast activity in both cancellous and cortical bone. Micro-CT analysis demonstrated marked improvements in bone microarchitecture, with bone mineral density increasing by 38.70% in the SST01 group and by 78.30% in the combined treatment group. Probiotic supplementation also increased fecal short-chain fatty acids, particularly butyrate and propionate, and reduced unloading-induced inflammatory cytokines, including TNF-α, IL-6, IL-1β, and IL-10. Whole-genome analysis confirmed the safety of both probiotic strains, with no classical virulence or acquired antimicrobial resistance genes detected. These findings indicate that S. thermophilus Yilife-SST01, especially when combined with B. bifidum BL-99, effectively alleviates unloading-induced osteoporosis by promoting bone formation, suppressing bone resorption, enhancing SCFA production, and modulating inflammatory responses, supporting its potential as a probiotic strategy for improving bone health.