Figure 46-1: (1) Reporter genes (e.g. bacterial luciferase, lux; firefly luciferase, luc; ice nucleation protein, ina) are introduced into phage genomes either by transpostion or via homologous recombination between wildtype phage and cloned phage sequences  carrying the reporter gene.  Recombinant phage are purified from the primary phage lysate by plaque purification and phenotypic screening for the expression of the reporter gene. (2) Purified Reporter phage (e.g. lux phage) are used to infect target cells (drawn as large shaded cell) in a mixed sample (other bacterial present represented by unshaded cells).  Phage DNA is injected into the host cell and replication begins. (3) Phage genes are expressed along with the inserted reporter gene.  The signal from the reporter gene (e.g. light) is detected indicating the presence of a cell which is sensitive to phage infection.