High-density and low-density capsule production byStreptococcus zooepidemicus

A group C streptococcus (Streptococcus zooepidemicus), isolated from the blood of a 79-year-old man who succumbed to infection, was examined for animal virulence, reactivity in an in vitro phagocytic assay, and the production of antiphagocytic structures. Initial examination of the clinical isolate by electron microscopy revealed the presence of high-density (HDC) and low-density (LDC) capsule variants. The HDC variant capsular material could be removed from the cell wall by hot acid extraction or obtained from the supernatant fluids when the organisms were grown in a defined medium. The capsular material was purified by ion-exchange chromatography on diethylaminoethyl cellulose and gel filtration on Sepharose 4B and was shown to be composed largely of a high molecular weight hyaluronic acid. Both the high-density and low-density capsule variants were shown to be mouse virulent with lethal dose 50 (LD₅₀) values of 3.7×10⁴ colony-forming units (CFU) and 3.3×10⁵ CFU, respectively. Opsonophagocytic assays with human peripheral polymorphonuclear leukocytes demonstrated that only the LDC organism could be phagocytized and killed in the presence of white blood cells, specific antibody, and complement.