Figure 3.

Mutant strains lacking production of SPN, SLO, or both toxins are attenuated in ability to cause tissue destruction in mouse models of necrotizing myositis and skin and soft tissue infection. A: Visual and microscopic examination of muscle lesions collected at 72 and 24 hours, respectively, after inoculation. The wild-type parental strain MGAS2221 causes large lesions (white boxed region, top left panel) with extensive destruction of the fascial tissues and myocytes (black boxed region, bottom left panel). Each isogenic mutant strain caused markedly smaller lesions (top panels) with less severe tissue damage that is confined to the fascial planes (bottom panels, inward facing arrows). B: Microscopic examination of skin lesions collected 72 hours after inoculation. The wild-type parental strain MGAS2221 caused large abscesses extending beyond the field of view (arrows) and ruptured through the skin surface (boxed region, left panel). Each single-gene isogenic mutant strain caused markedly smaller abscesses confined to the underlying soft tissue (arrows). The double-toxin deficient strain caused soft tissue infections without a discrete abscess. Hematoxylin and eosin staining was used. Original magnifications: ×4 (A); ×2 (B).