Rabbits immunized with group B, type Ia streptococci produce two distinct populations of protective antibodies. Evidence is presented indicating that these antibodies are directed against two major immunodeterminants which coexist in the specific type Ia antigen. Immunochemical data, using purified antibody preparations, indicate that the type substance, a sialic acid polymer consisting of galactose, glucose, glucosamine, and sialic acid, possesses two distinct immunodominant determinants, terminal sialic acid residues and a galactosyl oligosaccharide. Antibodies directed against either of these determinants were shown to possess in vivo and in vitro opsonic capabilities.

The in vitro interaction of a fresh clinical isolate of Staphylococcus aureus and polymorphonuclear leukocytes was investigated. The importance of the various cellular constituents as host immunological factors was analyzed, and the results suggested that two components, namely, an acidic polysaccharide consisting of a predominance of aminogalacturonic acid and a strain-specific mucopeptide complex, may be involved in impeding in vitro opsonization of the organism by leukocytes. Immunochemical analysis indicated that the acidic polysaccharide possessed the same immunodominant aminogalacturonic acid residues as the antiphagocytic acidic antigen of the encapsulated prototype Scott strain. Antisera derived from rabbits immunized with strain D contained two types of opsonins, those with acidic polymer specificity and those with mucopeptide complex specificity.

Two immunologically reactive polysaccharides have been isolated from the cell walls and from culture filtrates of a filamentous alpha-hemolytic streptococcus provisionally designated capsular type 83. Both polysaccharides were purified by diethylaminoethyl-cellulose chromatography. Analysis indicates that the capsular polysaccharide consists of galactose and phosphorus, whereas the cell wall polysaccharide contains galactosamine, glucosamine, glucose, and phosphorus. On the basis of immunochemical experiments, it is suggested that the capsular polysaccharide is composed of galactose-phosphate units with terminal galactose residues at the nonreducing end. It has also been found that the capsular antigen of streptococcus type 83 is shared by a number of streptococcal strains classified in Lancefield's group M. The cell wall polysaccharide of streptococcus type 83 cross-reacts with antibody to the C8, or cell wall-like capsular, polysaccharide of Diplococcus pneumoniae, and this cross-reactivity may be a reflection that the streptococcal antigen possesses certain structural features which are similar to those of pneumococcal C and C8 polysaccharides.

Mucopeptides isolated from Streptococcus bovis cell walls were found to be composed of alanine, glutamic acid, lysine, and threonine in a mole ratio of 3 :1:1:1. A dipeptide, NE-lysylthreonine, was isolated from S. bovis mucopeptide by ion-exchange chromatography. This finding suggests that threonine is associated with the bridge which cross-links adjacent tetrapeptides by connecting the e-amino group of lysine of one tetrapeptide to the carboxyl group of D-alanine of another to form the mucopeptide matrix.