Abstract
Lacticaseibacillus rhamnosus GG (LGG) is one of the most extensively studied probiotic strains, widely used in food and health applications. However, the absence of efficient, precise genome editing methods has limited its broader potential and functional versatility. Here, we present an endogenous type II-A CRISPR-Cas genome editing workflow for LGG designed for functional strain construction. Using a plasmid interference assay together with single-nucleotide substitutions, we confirm the precise PAM requirement as 5'-NGAAA-3'. We pair a synthetic sgRNA cassette with homology-directed repair donors to enable targeted deletions and insertions across multiple loci, achieving modest but practically relevant editing efficiencies (11.1-25.0% of recovered transformants) that support routine strain construction. Using this optimised and precise genome engineering method, we generated a β-glucuronidase (GUS)-expressing LGG strain for robust strain tracking within complex microbial communities. This work removes barriers to LGG engineering, expands the probiotic CRISPR toolkit, and provides broadly applicable strategies for designing next-generation probiotics with applications in food biotechnology and microbial therapeutics.