Possible common biological and immunological properties for detecting encapsulated strains of Staphylococcus epidermidis.

Twenty strains of capsular type II Staphylococcus epidermidis, determined by the method of Ichiman, were obtained from clinical specimens. Among them, 5, 5, and 10 strains were 4+, 3+, and 2+ in the intensities of their reactions against fluorescent antibody, respectively. Strains exhibiting 4+ and 3+ intensities were mouse virulent and phage nontypable, while 2+ strains were mouse avirulent and phage typable. When three strains randomly selected from each of the mouse-virulent and mouse-avirulent strains were compared in terms of their cell volume indices, all mouse-virulent strains had significantly higher indices (average, 1.86 times) than the mouse-avirulent strains. With intraperitoneal injection of the strains into mice, strains with higher cell volume indices resisted ingestion by peritoneal cells, while strains with low cell volume indices were sensitive to phagocytosis. When the capacity to absorb a definite amount of passive protective activity in rabbit antiserum prepared with capsular type II strains was compared among these strains, 10 to 20 mg of mouse-virulent strains was capable of completely absorbing the passive protective activity, whereas more than 80 mg of the cells was required for similar absorption by mouse-avirulent strains. In ultra-thin sections of three mouse-virulent strains stained with ferritin-conjugated rabbit antiserum, well-defined capsules were detected around cell walls; however, no capsule was seen around the walls of three mouse-avirulent strains.