Salmonella Rissen Φ1: a molecular switch

Introduction: Bacteria develop resistance against phages by losing the phage receptor or reducing its binding specificity or by a temporary change of phage receptor specificity. Here we describe the phage resistance mechanism adopted by S. Rissen which acts through a molecular switch. Material & Methods: Phage was isolated from the S. Rissen strain (RW) and used to select the phage resistant strain RRϕ1+. We evaluate both the differences in bacterial morphology and the genetic variations between the two strains by biochemical and comparative genomic analyses. Results: Biochemical analyses showed that the presence of the phage influences biofilm production, phage resistance and the switch of the O-antigen from smooth to rough, Genomic analysis showed that the sensitive and resistant strains differ by 10 genes. Only the phosphomannomutase_1 and 2 genes, involved in mannose synthesis pathway, showed different expression levels. The SNP of the two genes are located near HTs known to regulate phase variation. We used S rissen to see whether a character under strong selection pressure- such as phage resistance is repeatable. In four independent experiment phage resistance was acquired by the same molecular mechanism. Conclusion: S. rissen uses the same and evolutionary flexible tool (phase variation) to control several characters: biofilm production, phage resistance, and O-antigen structure.