Probiotic–Plant Bioactive Synergy in Gut Health: Mechanisms, Antimicrobial Activity, and Translational Challenges

Background/Objectives: Antimicrobial resistance (AMR), microbiota disruption, and chronic inflammation have intensified the search for alternative and complementary antimicrobial strategies. Probiotics and plant-derived bioactive compounds (phytochemicals) are increasingly being investigated as microbiota-supporting, immunomodulatory, and antimicrobial agents. This review synthesizes the current evidence on probiotic–phytochemical interactions, with particular emphasis on mechanisms relevant to antimicrobial synergy, gut barrier reinforcement, microbiota modulation, and translational development. Methods: A narrative literature review with a structured search strategy was conducted using major scientific databases, including PubMed, Scopus, EBSCO, Google Scholar, SpringerLink, Wiley Online Library, and Taylor & Francis, and open repositories. Publications from January 2016 to April 2026 were considered, with an emphasis on experimental, preclinical, clinical, and mechanistic studies addressing the combined use of probiotics, postbiotics, plant extracts, or defined phytochemicals. Results: Available evidence indicates that selected probiotic–phytochemical combinations may enhance antimicrobial activity through complementary mechanisms, including pathogen membrane destabilization, inhibition of adhesion and biofilm formation, quorum-sensing interference, stimulation of probiotic viability and metabolite production, and biotransformation of phytochemicals into more active derivatives. These interactions may also support epithelial barrier integrity and immune regulation. However, the evidence remains heterogeneous and is strongly influenced by probiotic strain identity, phytochemical composition, dose, formulation, and the experimental model. Most studies are still limited to in vitro or animal models, and clinical validation remains scarce. Conclusions: Probiotic–phytochemical combinations represent a promising but insufficiently standardized strategy for antimicrobial and microbiota-targeted interventions. Future progress requires chemically characterized plant preparations, strain-level probiotic selection, harmonized synergy assays, advanced delivery systems, and well-designed clinical trials.