Assessment by a fluorochrome microassay of phagocytic killing of group B streptococci adherent to glass

Abstract

An in vitro adhrence assay was developed to study the interaction between bovine polymorphonuclear cells (PMN) and group B streptococci (GBS) on glass. The kinetics of uptake and killing of GBS in the presence and absence of opsonins were compared by staining bacteria with a fluorochrome (acridine orange). Opsonization was not required for the PMN to ingest and kill glass-adherent GBS. Nevertheless, the presence of serum during the phagocytosis incubation, or preopsonization of bacteria, both increased the rate of ingestion and the number of bacteria per PMN at the end of the reaction (90 min). Precolostral calf serum (PCS), almost devoid of antibodies, was as effective as normal bovine serum in this respect. By contrast, PCS was not an effective opsonin source when phagocytosis took place in suspension, demonstrating further that different underlying mechanisms operate depending on the physical presentation of bacteria to phagocytes. It was noted that PMN monolayers were able to ingest unopsonized bacteria that merely settled on the glass surface during phagocytosis incubation. The method should permit investigations on the mechanisms involved in surface phagocytosis, a defense mechanism of potential importance in local infections.