From lab to law: emerging applications, potential benefits, evolving regulatory framework and challenges for engineered probiotics.

Engineered probiotics are emerging as versatile biological platforms capable of delivering therapeutic functions, modulating host–microbiota interactions, and enabling innovative strategies for preventing or treating metabolic, infectious, and inflammatory conditions. Advances in synthetic biology have expanded microbial engineering along a continuum ranging from self-cloned or intragenic modifications—based on deletions or recombination events that recapitulate naturally plausible genomic changes—to fully transgenic constructs expressing heterologous bacterial, viral, or human genes. This technological diversity demands proportionate and mechanistically informed safety evaluation, with particular emphasis on genetic stability, ecological compatibility, and the potential for horizontal gene transfer (HGT). This review examines the principal applications of engineered probiotics in human health, including strains designed to enhance endogenous functions, eliminate detrimental activities, neutralize toxins, interfere with pathogen signaling, degrade biofilms, express therapeutic proteins, act as mucosal vaccine platforms, serve as tumor-targeted immunotherapeutic vectors, or enable emerging systemic and brain-directed delivery strategies. We also highlight the current regulatory heterogeneity across international frameworks and discuss the relevance of recent EFSA guidance, which clarifies that modifications involving only deletions or the reinsertion of native sequences may entail markedly different regulatory obligations compared with constructs carrying truly novel genetic traits. To promote regulatory convergence, we propose a unified safety-assessment framework that integrates classical toxicological testing with a construct-specific evaluation of HGT potential. This approach combines whole-genome sequencing to define the engineered locus, validated qPCR assays for highly specific detection, and controlled exposure experiments using competent microbiota and environmental recipient strains to quantify the extremely low probability of gene transfer under worst-case conditions. Such a structured methodology provides a scalable, evidence-driven basis for evaluating engineered probiotics according to the biological nature of the modification rather than a one-size-fits-all model. Engineered probiotics hold substantial translational promise, provided that safety assessments remain adaptive, risk-proportionate, and aligned with mechanistic understanding of microbial genetics and ecology.