Unlike lytic phages, temperate phages maintain long-term association with their bacterial host through lysogeny. In this context mutual beneficial interactions can evolve that support efficient co-reproduction. We recently identified a new type of bacteria-phage interaction in which a prophage integrated within a critical bacterial gene serves as an active regulatory switch to regulate the gene expression via genome excision, a phenomenon we termed active lysogeny. Regulation of comK gene in the human pathogen Listeria monocytogenes is mediated via phage genome excision and re-integration. ComK itself is the activator of the competence system, which is necessary to promote L. monocytogenes infection of mammalian cells. Interestingly, prophage excision is specifically induced during L. monocytogenes intracellular growth, primarily within the mammalian cells phagosomes, yet, unlike classic prophage induction, progeny virions are not produced. This study evidences a unique bacterial-phage adaptation that turns the prophage into a genetic switch that plays an important role in the intracellular lifestyle of its host.