A Minimal and Portable CRISPR Platform Based on Bifidobacterial Cas9 Enables Genome Editing in E. coli Nissle 1917

Genetic manipulation of core gut probiotics remains challenging due to endogenous cellular barriers and a scarcity of efficient molecular tools, limiting progress in live biotherapeutic development. Here, we characterized the native type II-C CRISPR-Cas system in Bifidobacterium longum subsp. longum GNB (B. longum GNB). Through integrated bioinformatic analysis and high-throughput protospacer adjacent motif (PAM) screening, we identified a novel 5'-NNRMAT-3' (where R = A/G, M = A/C) motif recognized by its compact Cas9 nuclease (BLCas9). The stringent PAM dependency of BLCas9 was unequivocally confirmed by in vitro cleavage assays. Leveraging this endogenous mechanism, we developed a dual-plasmid editing platform for robust and multiplex genome engineering in the probiotic strain Escherichia coli Nissle 1917 (EcN). Application of this system notably enhanced extracellular γ-aminobutyric acid (GABA) production in EcN through targeted metabolic engineering. Our work provides the first molecular dissection of a type II-C system in Bifidobacterium longum and establishes a generalizable framework for the discovery and application of compact programmable nucleases, suggesting a viable strategy for modulating host physiology via the gut-brain axis.