Modifying bacteriophage

A method for modifying the genome of a lytic target phage, wherein the target phage genome includes a first target sequence and a second target sequence, which method comprises:

• (a) providing a vector which contains a phage-targeting region comprising a host range determinant of a marker phage, different from the host range determinant of the target phage, wherein the phage-targeting region is flanked by first and second flanking sequences homologous to the first and second target sequences of the target phage genome, and wherein the phage-targeting region of the vector further comprises an exogenous DNA sequence for incorporation into the genome of the target phage;
• (b) introducing the vector into a first host cell, which host cell is a host for the target phage;
• (c) infecting the first host cell with the target phage;
• (d) allowing replication of the phage and recombination between the target phage genome and vector to take place whereby the genome of the target phage is modified;
• (e) propagating resultant phage on a second host cell, which host cell is a host for the marker phage and not the target phage, wherein said propagating indicates that said resultant phage comprise said exogenous DNA sequence;
• and
• (f) harvesting the resultant phage.