This study demonstrates the successful in situ targeted base editing of bacteria in the mouse gut using phage-derived particles. The researchers engineered a phage particle to deliver a base editor and modify *Escherichia coli* colonizing the mouse gut. The editing of a β-lactamase gene in a model *E. coli* strain resulted in a median editing efficiency of 93% of the target bacterial population with a single dose. The edited bacteria were stably maintained in the mouse gut for at least 42 days following treatment. The non-replicative DNA vector prevented the maintenance and dissemination of the payload. The researchers then applied this approach to edit several genes of therapeutic relevance in *E. coli* and *Klebsiella pneumoniae* strains in vitro and demonstrated in situ editing of a gene involved in the production of curli in a pathogenic *E. coli* strain. This work shows the feasibility of modifying bacteria directly in the gut, offering new avenues to investigate bacterial gene functions and develop microbiome-targeted therapies.This study demonstrates the successful in situ targeted base editing of bacteria in the mouse gut using phage-derived particles. The researchers engineered a phage particle to deliver a base editor and modify *Escherichia coli* colonizing the mouse gut. The editing of a β-lactamase gene in a model *E. coli* strain resulted in a median editing efficiency of 93% of the target bacterial population with a single dose. The edited bacteria were stably maintained in the mouse gut for at least 42 days following treatment. The non-replicative DNA vector prevented the maintenance and dissemination of the payload. The researchers then applied this approach to edit several genes of therapeutic relevance in *E. coli* and *Klebsiella pneumoniae* strains in vitro and demonstrated in situ editing of a gene involved in the production of curli in a pathogenic *E. coli* strain. This work shows the feasibility of modifying bacteria directly in the gut, offering new avenues to investigate bacterial gene functions and develop microbiome-targeted therapies.