Protection of mice by a pseudodiffuse strain of Staphylococcus aureus possessing polyvalent capsular type antigen.

Staphylococcus aureus strain MC31 showed pseudodiffuse growth in serum-soft agar and reacted with immune rabbit sera to strains Smith diffuse (capsular type A), NS58D (type B) and NS41D (type C) but not with strain NS68D (type D) in serum-soft agar. Immunisation of mice with 300 micrograms of cell-surface polysaccharide extracted from strain MC31 protected against lethal infection by strain MC31 and the strains of capsular types A, B and C. Immune rabbit serum prepared against strain MC31 passively protected mice against challenge infection with the homologous strain, but approximately 30 times more anti-MC31 serum was required to protect against infection with the strains of capsular types A, B and C. Absorption of the passive protective activity of immune sera raised against the three capsular type strains required at least 10 times the quantity of MC31 cell-surface polysaccharide than the quantity of cell-surface polysaccharide from the homologous capsular strain. Electronmicrographs of strain MC31 treated with ferritin-labelled antisera to the three capsular strains showed only small amounts of ferritin granules around the cell wall.