Relation of colonial morphologies of strain of Streptococcus pneumoniae in soft-agar to the encapsulation

Among 40 fresh isolates of Streptococcus pneumoniae 12, 25 and 3 strains, respectively, exhibited large round, small round and compact colonial morphologies in soft-agar medium. Every large round strain possessed a capsule, almost half of the small round strains had capsules, while all of the large round type growth showed very high mouse virulence and 1.0 mg of these organisms was capable of absorbing a minimal amount of passive protective antibody in rabbit antiserum, prepared with the homologous strain, against challenge infection with homologous organisms in mice. Its variant showing compact type growth in soft-agar was mouse avirulent and a similar amount of the mouse passive protective antibody could not be absorbed with 100 mg of these organisms. These experimental results indicate that the soft-agar technique can be used for the identification of encapsulated strains of Streptococcus pneumoniae.