The Saudi Journal of Medical Insights

The human gut microbiome is a critical mediator of health and disease, yet conventional probiotics face significant
limitations, including poor survival, transient colonization, and pleiotropic, often unelucidated, mechanisms of action.
Synthetic biology, which applies engineering principles to biology, is revolutionizing this field by enabling the rational
design of next-generation probiotics as precise "living therapeutics." This narrative review delineates the paradigm shift
from conventional supplementation to the engineering of probiotics with enhanced functionalities. A thorough search was
performed across major academic databases, including PubMed, Scopus, and Web of Science, utilizing key search terms
such as "synthetic biology," "engineered probiotics," "CRISPR," "genetic circuits," "gut microbiome," and "live
biotherapeutics." We explore the advanced synthetic biology toolkit—including CRISPR-based genome editing, genetic
circuit design, and biocontainment strategies—that allows for the reprogramming of bacterial chassis. Key applications are
highlighted, such as engineered probiotics for the treatment of metabolic disorders through the in situ production of
therapeutic peptides like GLP-1, and for inflammatory bowel disease (IBD) via sense-and-respond circuits that detect
inflammatory biomarkers and deliver anti-inflammatory cytokines. Furthermore, we detail the mechanisms by which
probiotics, both conventional and engineered, influence gut health, including pathogen inhibition, epithelial barrier
reinforcement, immunomodulation, and bioactive metabolite production. Finally, we discuss the design of probiotics with
enhanced survivability, niche colonization, and targeted antimicrobial capabilities. While challenges in safety and regulation
remain, synthetic biology heralds a new era in gut health, offering a powerful platform for developing personalized,
effective, and safe live biotherapeutic products. Long-term safety in humans remains under investigation.