Change in phage-sensitivity of Shigella flexneri strains. I. Changes in virulence and phage-sensitivity

In the course of repeated passaging of virulent Shigella flexneri cultures in liquid media, avirulent variants and variants of reduced virulence appeared. They were easily recognizable by their colonial morphology. The avirulent variants became sensitive to certain phages not lysing the original virulent strains. The change of phage-sensitivity among the variants with no or decreased virulence was significant: chi 2 = 64.25; P less than 0.001. The amplification of phage-sensitivity developed in 15% of the originally avirulent cultures. No association was found between the loss of virulence and a specific phage-sensitivity pattern. In the majority of the cases sensitivity to phage Ms2 developed during the passages, but it was observed in some of the original strains, too. With few exceptions, sensitivity to phage Ms2 was associated with the total or partial degradation of the specific antigen and the antigenic structure detectable by S. flexneri factor sera. These exceptions were cultures with maintained or decreased virulence.