Identification and Isolation of Lysogens with Induced Prophage

The fate of lysogens following prophage induction has assumed added significance with the finding that in many pathogens virulence genes are carried on prophages and, in some, the production and/or release of the virulence factor is under control of the phage lytic regulatory program. We outline a method for identifying and characterizing from a total lysogen population, the subpopulation in which the prophage is induced. The prophage is genetically altered so that on induction it does not go through the lytic pathway, but does express a resolvase that acts at a reporter cassette located at another site on the bacterial chromosome to irreversibly change the resistance of the bacterium from tetracycline to chloramphenicol. Thus, induced derivatives survive and are easily identified even if they make up a small fraction of the population.