The inhibition of surface phagocytosis by the capsular slime layer of pneumococcus type III

Five strains of type III pneumococcus have been shown to possess wide capsular slime layers during the logarithmic phase of growth in serum broth. The slime layer stains metachromatically with methylene blue and can be visualized under the electron microscope as a fuzzy halo which extends well beyond the surace of the capsule proper and causes centrifugates of the organism to be of extremely large volume. This outer capsular structure is most readily demonstrated in vivo and in nutrient broth containing glucose and serum. It disappears from the surface of the cell with aging of the culture, and is easily removed by dilute alkali, alcohol, and heat. Exposure of slime-covered type III pneumococci to homologous antibody and to type III polysaccharidase reveals that the slime layer contains the same type-specific polysaccharide that is present in the rest of the capsule. From a type III strain producing a prominent slime layer an intermediate mutant has been isolated which forms small non-mucoid colonies on blood agar and possesses a relatively small capsule with a barely discernible slime layer. The wide slime layer protects virulent type III pneumococci from surface phagocytosis. Whenever the type III cells lose their broad slime layer, whether from aging of the culture, from mutation, from exposure to injurious chemicals, or from the action of type III polysaccharidase, they become susceptible to phagocytosis by the surface mechanism. Once phagocyted the type III pneumococci are promptly destroyed, even in the absence of antibodies.