Restriction and modification of Shigella flexneri phages by R factors.

Out of 420 R factors derived from Shigella flexneri strains, 50.8% restricted Escherichia coli and S. flexneri phages. Phage restriction was produced both by fi- and fi+ R factors. The R factors were divided into nine groups on the basis of the efficiency of plating of S. flexneri phages. Changes of phage types were produced by transferring R factors of different restrictive types. The changes offered some information concerning the evolution of phage types. Studies on phage modification supported the grouping of R factors determined on the basis of restriction. R factors of different restrictive types were type-specific except for types VII and IX. Modified phages proved to be highly practical for epidemiological purposes. The use of modified phages, as an additional phage-set besides that basic phage-set, was suggested to trace the source of strains which changed their phage types as an effect of R factors.