Role of phage Φ1 in two strains of Salmonella Rissen, sensitive and resistant to phage Φ1

Background: The study describes the Salmonella Rissen phage Φ1 isolated from the Φ1-sensitive Salmonella Rissen strain R. The same phage was then used to select the resistant strain RΦ1+, which can harbour or not Φ1. Results: Following this approach, we found that Φ1, upon excision from R cells with mitomycin, behaves as a temperate phage: lyses host cells and generates phage particles; instead, upon spontaneous excision from RΦ1+ cells, it does not generate phage particles; causes loss of phage resistance; switches the O-antigen from the smooth to the rough phenotype, and favors the transition of Salmonella Rissen from the planktonic to the biofilm growth. The R and RΦ1+ strains differ by 10 genes; of these, only two (phosphomannomutase-1 and phosphomannomutase-2; both involved in the mannose synthesis pathway) display significant differences at the expression levels. This result suggests that phage resistance is associated with these two genes. Conclusions: Phage Φ1 displays the unusual property of behaving as template as well as lytic phage. This feature was used by the phage to modulate several phases of Salmonella Rissen lifestyle.