Escherichia coli Nissle 1917: Molecular Determinants of Probiotic Activity, Safety, and Therapeutic Potential

Escherichia coli Nissle 1917 (EcN) is one of the most intensively investigated Gram-negative probiotic strains that have been clinically used for over a century. However, how EcN’s strain-specific traits contribute to its probiotic activity, safety profile, and therapeutic potential remains poorly understood. In this review, we link its clinical history and unique microbiological and genomic characteristics to the bioactive molecules and cell-surface structures responsible for its probiotic activity. These include antimicrobial microcins and siderophore-microcins, iron-acquisition systems, and locally acting metabolites involved in nutrient competition and colonization. They also include outer membrane vesicles and surface structures, such as lipopolysaccharide, flagella, fimbriae, and capsular polysaccharides, that mediate host interactions, barrier protection, and immune modulation. Together, these factors support intestinal homeostasis and distinguish EcN from pathogenic E. coli. By controlling pathogens through non-antibiotic mechanisms, EcN may also support antibiotic-sparing approaches that can help prevent antimicrobial resistance. However, many molecular functions are supported primarily by in vitro and animal studies, and their contribution to clinical efficacy remains less well established. We also discuss safety considerations, particularly the pks/colibactin island, genotoxicity, and long-term therapeutic use. By linking EcN’s clinical relevance to its molecular traits, we highlight how mechanistic understanding can support next-generation probiotics and live biotherapeutic products.