Comparison of Compact and Diffuse Variants of Strains of Staphylococcus aureus

Several biological characteristics of six diffuse-type variants and six compact-type variants of Staphylococcus aureus were compared on the basis of morphology, phage type, certain cellular products, growth rate, and mouse virulence. All compact variants gave a positive test for clumping-factor reaction, coagulase, deoxyribonuclease, mannitol fermentation, hemolysis, and pigmentation. Four of the six compact variants were phage typable and lacked capsules. None of the diffuse variants was phage-typable or possessed the clumping-factor reaction. Only one diffuse variant had hemolytic activity, and all of the diffuse strains were encapsulated. No differences between the compact and diffuse strains were noted with respect to mannitol fermentation or deoxyribonuclease activity. Coagulase and acid phosphatase activities of the culture supernatant fluid were diminished in most of the diffuse strains. Less beta-carotene content was found in cells of diffuse variants. Virulence in mice was found to correlate with the capsule size, as the mortality rate was greatest for diffuse variants having the largest capsules. Growth rates of variants were generally not significantly different.