MEASURING RESISTANCE TO PHAGOCYTOSIS OF GROUP-A AND GROUP-G STREPTOCOCCI - COMPARISON OF DIRECT BACTERICIDAL ASSAY AND FLOW-CYTOMETRY

M protein is thought to contribute to the ability of non-opsonized gro up A and group G streptococci (GAS and GGS, respectively) to resist ph agocytosis by polymorphonuclear leukocytes. In previous studies, corre lation between M protein expression and phagocytosis was determined by incubating these pathogens in human blood and comparing colony-formin g bacterial counts prior to and after exposure to blood (direct bacter icidal assay; DBA). Here, we report the application of flow cytometry to measure GAS and GGS resistance to phagocytosis. The results of the assays were in complete agreement with those from DBAs. Nevertheless, flow cytometry was regarded as superior to DBA because of its speed an d potential uses for quantitative studies. In addition, the use of ant i-CD 11b monoclonal antibody for granulocyte staining guaranteed a non -compromized granulocyte function. The optimized protocol for now cyto metry presented here could be utilized to directly measure the involve ment of individual protein types in bacterial resistance to phagocytos is.