Correlation between the virulence of streptococci for mice and their IgG FcR activity in in vivo experiments

The immunological analysis of 24 spontaneous Strr, Rifr and Kanr mutants of streptococcal strain IP, highly virulent for mice and capable of binding polyclonal human IgG (IgG FcR+), was made. The characteristic feature of all these mutants was decreased virulence, restored after their passage in vivo. 23 mutants were capable of binding polyclonal IgG; one Strr mutant had no such capacity, but acquired it, together with an increase in virulence, after its passage in vivo. When stored in meat-peptone agar without antibiotics, 5 out of 10 Strr mutants lost their capacity for binding polyclonal human IgG. After passage in vivo they regained this property simultaneously with virulence.