No abstract

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.

In the pncumonic lung of the rat the leucocytes of the alveolar exudate phagocyte virulent pneumococci in the apparent absence of type-specific antibodies (I, 2). This finding runs counter to the present teachings of immunology since numerous investigators bare presented evidence that fully encapsulated pneumococci are resistant to phagocytosis except when coated with a suitable opsonin (3-5). Only Goodner and Miller (6) have reported appreciable phagocytosis in the absence of antibody and they have expressed doubt as to the significance of such phagocytosis, since the non-sensitized pneumococci appeared, in their experiments, to multiply within the phagocytes. The experiments already described in an accompanying paper indicate that the phagocytosis observed in the pneumonic lung during treatment with sulfonamide is due neither to the presence of type-specific opsonins in the lung nor to a direct effect of the drug upon the pneumococcus capsule (2). It would appear, therefore, that the destruction of pneumococci in the lungs is duc to other factors hitherto not understood. The nature of these factors is defined in the present paper?It has been shown in previous experiments that pneumococci inoculated upon a resolving pneumonic lesion arc quickly phagocyted and destroyed by the phagocytic cells in the alveolar exudatc. Although evidence has been presented that the phagocytosis is not due to the presence of type-specific opsonins, it is still conceivable that the phagocytic reaction in the living animal is dependent upon unknown factors which (I) are concentrated in the pneumonic exudate, (2) are brought to the alveoli by the circulating blood, or (3) are present in living pulmonary tissue. The following experiments would appear to eliminate all such hypothetical "phagocytic factors" originating from any one of these three possible sources.