Lancefield group C Streptococcus milleri group strains aggregate human platelets

Lancefield group C Streptococcus milleri group (SMG) strains are the only SMG types that are able to aggregate human platelets. Complete aggregation occurred within 10 min of mixing bacterial cells and platelets together in the ratio 8:1. Substances which (i) chelated cations; (ii) inhibited the cycloxygenase pathway in platelets; (iii) reduced the availability of ADP and disrupted platelet membrane stability; (iv) reduced bacterial aggregation of platelets. The platelet-interacting substance on the surface of the SMG appeared to be proteinacious as digestion of bacterial cells with protease inhibited aggregation whereas treatment with lipase, periodate or antisera to Lancefield group C polysaccharide had no effect.