Defect-complementation homologous recombination: A novel strategy for precise genome engineering of virulent phages

Engineered bacteriophages (phages) have been developed to overcome the limitations of natural phage therapies and serve as precision-targeted agents against drug-resistant bacterial infections. However, their application has been constrained by the low efficiency of existing genome-editing tools, largely because of the absence of effective selection markers. This study proposed a novel strategy, termed defect-complementation homologous recombination (DCHR), for precise phage genome editing. In this approach, CRISPR-Cas9 cleaves a donor plasmid in host cells to release a linear donor template carrying homology arms, an essential phage gene used as a selection marker, and two <i>lox</i> sites. The donor template undergoes homologous recombination with the genome of essential gene-deficient phage, thereby enabling targeted genome modifications. Using DCHR, we successfully generated large genomic deletions (1.48-kb <i>gp0.4-0.7</i> and 1.02-kb <i>gp4.3-4.7</i>), achieved gene insertion (3.08-kb <i>lacZ</i>), and introduced a single-base substitution (TGA to TAA) in the stop codon of <i>gp9</i> within the same T7 phage genome, all with 100 % accuracy. The significant advantages of DCHR are as follows: (i) High-efficiency screening: Only progeny phages derived from successful homologous recombination retain viability and replicative capacity, thereby greatly simplifying recombinant isolation. (ii) Editing flexibility: Unlike CRISPR-Cas systems, DCHR cannot be constrained by protospacer adjacent motif dependence and allows modifications across diverse genomic loci. (iii) High recombination efficiency: DCHR can achieve a recombinant phage titer of 3.1 × 10⁵ PFU mL^-1 (plaque-forming units per mL) without relying on exogenous homologous recombination systems. In summary, DCHR demonstrates potential as a precise and efficient general genome-editing tool that facilitates design of engineered phages and advances functional genomic studies.