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Repeated passage of freshly isolated group A streptococci on blood agar. I. Effect on M protein, opacity factor and IgG Fc-receptor activity.

Seventy-six strains, 20 T-type 1, 20 T-type 2, 18 T-type 4 and 18 T-type 12 group A streptococci were isolated from throat cultures and subjected to 25 serial passages on blood agar. A single "glossy" colony was selected from each passage in order to diminish the M protein content of the strains. In accordance with previous results, the M protein synthesis, as estimated by an electro-immuno assay diminished in the T1 strains, in mean from 23.8 to 15.5% of a reference M protein preparation (p less than 0.01). Opacity factor (OF)-production was used as a measure for M protein in the T-type 2, the T-type 4 and the OF-positive T-type 12 strains. OF-excretion decreased significantly in the T-types 2 and decreased, though not significantly in the T-type 12 strains but increased (unexpectedly) in the T-type 4 isolates during subculture. However, irrespective of the changes in M protein/OF production these types all increased significantly in capacity to bind radiolabelled IgG via the Fc-fragment. It is known that streptococci isolated during convalescence resemble subcultured strains; it is suggested that IgG Fc-receptors are important for protection of the streptococci against phagocytosis in the asymptomatic carrier state.