Lacticaseibacillus rhamnosus AC1 Aggravates Bone Loss in a Male Rat Model of Deoxycorticosterone Acetate (DOCA)-Salt-Induced Osteoporosis

<b>Background/Objectives</b>: Osteoporosis is a prevalent and debilitating skeletal disease characterized by a progressive loss of bone mass and deterioration of bone microarchitecture. Probiotics have emerged as a potential therapeutic tool for treating osteoporosis through modulation of the gut microbiota. In this study, we aimed to examine the effects of live <i>Lacticaseibacillus rhamnosus</i> AC1 (LR-AC1), isolated from a fecal sample from a newborn in Hong Kong, on deoxycorticosterone acetate (DOCA)-induced bone loss in a rat model. <b>Methods</b>: Bone mass and microarchitecture were assessed using micro-computed tomography (micro-CT). Immunostaining for CD31⁺ and osterix, markers of endothelial cells and osteoblast precursors, respectively, was performed. Gut microbiota composition was analyzed via 16S rRNA sequencing. The effects of an LR-AC1 cell-free conditioned supernatant (CCS) on osteoclastogenesis, angiogenesis, and migration of bone marrow mesenchymal stem cells (BMSCs) were evaluated in vitro using RT-qPCR and wound healing assays. <b>Results</b>: LR-AC1 administration did not induce adverse effects in healthy rats; however, it exacerbated bone loss in rats with DOCA-salt-induced osteoporosis. Correspondingly, the number of CD31-positive endothelial cells and osterix-positive osteoprogenitors decreased with bone loss. In vitro, LR-AC1 CCS promoted osteoclastogenesis and angiogenesis, while in the presence of DOCA, LR-AC1 CCS inhibited BMSC migration. Gut microbiota analysis revealed that the relative abundances of the genera <i>g_RF39</i> and <i>g_Clostridia_UCG-014</i> correlated with the severity of bone loss. <b>Conclusions</b>: While several studies suggest that probiotics can prevent and treat osteoporosis, our findings indicate that in a male rat model of DOCA-salt-induced osteoporosis, live LR-AC1 aggravated bone loss. This effect is associated with alterations in gut microbiota and disruption of the coupling process in bone remodeling.