Role in virulence of Shigella flexneri antigens derived from lysogenic conversion.

The virulence of Shigella flexneri var. Y (NTCC 4839), its lysogenic convertants characterized by antigens I, V, and 7, 8, and of a phage-free clone selected by means of antiphage serum was studied. The parent strain was avirulent in the guinea pig eye test and in the "mouse shigellosis" model, but chicken embryo tests indicated the presence of penetrating ability together with defective tissue-growing capacity. The lysogenic convertants failed to regain their virulence to the guinea pig eye and to the mouse, but showed an increased tissue-growing capacity for chicken embryos. The level of virulence of the phage-free derivative equaled that of the parent strain. We concluded that the terminal glucose component of O antigen, even of conversion origin, plays a role as one of the virulence factors in tissue-growing capacity.