<p>The oral microbiome plays an important role in maintaining peri-implant health, and microbial imbalance is a key factor in the development of peri-implant mucositis (PIM). Probiotics have been proposed as a strategy to modulate the oral microbiome, but clinical evidence in peri-implant conditions remains limited. In this randomized exploratory trial, patients with PIM received the probiotic Weissella cibaria CMU (OraCMU) or placebo for four weeks following standard supragingival scaling and oral hygiene instruction. Subgingival microbiome profiles and clinical parameters were evaluated at baseline and follow-up. While overall microbial diversity showed no marked differences between groups, OraCMU administration was associated with favorable compositional changes within the subgingival microbiome. This included enrichment of bacterial taxa commonly associated with peri-implant health and suppression of taxa linked to inflammatory peri-implant conditions. Consistent trends toward reduced abundance of key periopathogenic species were observed in the probiotic group compared with placebo. A modest improvement in probing depth was also noted in patients with moderate PIM receiving OraCMU. Together, these findings indicate that short-term probiotic supplementation may support ecological stabilization of the peri-implant microbiome. Further longitudinal studies are needed to determine whether such microbiome modulation translates into sustained clinical benefit.</p>