Measuring resistance to phagocytosis of group A and G streptococci: comparison of direct bactericidal assay and flow cytometry.

M protein is thought to contribute to the ability of non-opsonized group A and group G streptococci (GAS and GGS, respectively) to resist phagocytosis by polymorphonuclear leukocytes. In previous studies, correlation between M protein expression and phagocytosis was determined by incubating these pathogens in human blood and comparing colony-forming bacterial counts prior to and after exposure to blood (direct bactericidal 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 and potential uses for quantitative studies. In addition, the use of anti-CD11b monoclonal antibody for granulocyte staining guaranteed a non-compromized granulocyte function. The optimized protocol for flow cytometry presented here could be utilized to directly measure the involvement of individual protein types in bacterial resistance to phagocytosis.