Factors causing the clumping reaction of streptococcal strains with human plasma.

Fresh isolates of 204 strains of Streptococcus haemolyticus, 75 strains of viridans group Streptococcus, and 45 strains of Streptococcus pneumoniae were studied for their clumping reactions with human plasma. The plasma and serum factors that clumped the streptococcal strains were compared with those that clumped a Staphylococcus aureus strain. One hundred eleven strains of S. haemolyticus, 10 strains of viridans group Streptococcus, and none of the strains of S. pneumoniae tested were clumped by human plasma. However, clumping activity was remarkably unstable after subculturing on plates containing Todd-Hewitt blood agar. The strain with the highest level of activity was clumped by fibrinogen and normal human serum. Results of tests of the clumping reactions of staphylococci and streptococci with human serum indicated that the serum factors responsible for those reactions may be identical.