Capsule Production and Virulence among Strains

In 1956, just after Price and Kneelandl first reported the occurrence of a specific capsular reaction in a strain of mucoid S . aureus isolated after egg passage of a throat isolate with an A strain of influenza virus, we were searching about for an area of endeavor that might interest us. Their initial report was followed by a second one in which they seemed to downgrade the importance of their finding by reporting what they interpreted as a lack of specificity, owing to the fact that all of 39 isolates of S . aureus from clinical material exhibited specific capsular reactions with a single antiserum. We could not agree with their conclusion; this proved to be the stimulus that awakened our interest in the occurrence of encapsulation and its significance among strains of S. aureus. Our first efforts in the field were concerned with the isolation of mucoid variants of S. aureus; we employed the technique of Bigger and colleagues.:: They cultivated staphylococci in broth that contained a fermentable carbohydrate, and maintained the cultures in this environment for up to two months. Periodic subcultivation to agar plates and subsequent testing of colonies for mucosity was used as a screening proceduce to obtain mucoid variants. With this procedure we obtained five mucoid strains of S. aureus. One strain, which originated from an infected sutured wound, was especially mucoid and was used to stimulate antibody production in rabbits and roosters. A long and vigorous immunization course was used, as described by Alexander and colleagues for the production of antiserum for use in the specific capsular reaction with Haetnophilus influenzae. A positive specific capsular reaction obtained with the rooster hyperimmune serum and a negative reaction are shown in FIGURES 1A and 1B respectively. A tendency to decry the finding of capsules on mucoid variants of S. aureus obtained by laboratory manipulation led to studies of the incidence of naturally occurring strains of encapsulated S. nureus and the natural occurrence of anticapsular antibodies. Healthy human blood donors from a rural area submitted to swabbing of their anterior nares and also donated samples of their plasma. The swabs were planted on mannitol salt plates and colonies were picked and placed in buffered nutrient glycerol broth for subsequent testing in the specific capsular reaction, in which we used a hyperimmune rabbit antistaphylococcus serum. We used the plasma samples of the donors in a specific capsular reaction test, using the known encapsulated S. aureus we had isolated. The results of these tests are shown in TABLE 1. Of the 109 blood donors, 43% carried S .