Biological and Immunological Properties of Encapsulated Strains of Staphylococcus aureus from Human Sources.

Of 875 strains of Staphylococcus aureus isolated from human source clinical specimens, 37 (4.2%) were encapsulated strains. These were all negative for clumping factor and could not be typed with bacteriophages or by serology. Twenty-one of these did not produce any hemolysins, 15 produced alpha hemolysin, 1 produced beta hemolysin, and 1 produced both beta and delta hemolysins. After one or two subcultures, 27 of the encapsulated strains converted to the compact variant form, all became positive for clumping factor, 12 became phage-typable, and 24 became sero-typable. In addition, 7 strains converted from negative to alpha hemolysin production. Comparison of phage- and sero-types did not reveal any relationships. Immunologically, mice challenged with heat-killed encapsulated strains were protected against a challenge infection with the Smith diffuse strain. Protective antibodies in rabbit anti-Smith diffuse strain antisera were removed by adsorption using the encapsulated organisms isolated in this study. The adsorbed sera no longer protected against challenge infection in mice with the Smith diffuse strain. From these results, it appears that the encapsulated strains isolated were immunologically and biologically similar to the classical Smith diffuse strain.