Viral spread within ageing bacterial populations.

The viral spread within isolated host populations has been studied throughout the growth of P22-infected Salmonella cell colonies. By using an integration mutant of this bacteriophage, horizontal and vertical transmission have been analyzed independently. The data obtained show that both strategies are not simultaneous but consecutive during the colony development. Lytic cycles are tightly repressed during the exponential cell growth but stimulated in independent colonies with remarkable synchrony when the cell division rate decreases. The coincidence of the viral outburst and the decay of bacterial replicative fitness is a new example of the extreme viral competence in exploiting the host cells as dissemination vehicles for viral genomes.