A screening of streptococci freshly isolated from human and animal sources for binding of human IgG

Human isolates of groups A, C and G streptococci as well as animal isolates of group C were investigated with respect to their binding capacity for human IgG by using the direct fluorescence technique and the Mancini test. From each serological group of human isolates, more than 100 strains were tested. The results were evaluated statistically with respect to serological group, type and source of isolates. Between human isolates of groups A, C and G, no statistical differences concerning IgG binding were found. However, group C streptococci isolated from pigs showed a significantly higher number of strains with high IgG binding than the human isolates of groups A, C and G. Group A streptococci isolated from suppurating lesions showed an increased IgG uptake when compared with isolates from scarlet fever patients or patients with throat infections. However, strong IgG binding by group A streptococci seems not to be restricted to certain types. By using selected streptococcal strains, it was found that IgG absorption from a solution of purified IgG was much higher than from IgG solutions containing albumin or from diluted serum. The results are discussed in connection with the competition of different plasma proteins for binding sites on the streptococcal cell surface and with a possible influence of IgG receptors on the virulence of streptococci.