598 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



uals. Well developed capsules which may surround the single organism or the pairs 

 and chains may be found in exudates or in milk and serum media. There are no 

 spores nor flagella. The cocci stain readily with the aniline dyes and are Gram- 

 positive. The capsule can be demonstrated by several methods of which Welch's and 

 Hiss' are the most common. The temperature range is from 25 to 41. It is both 

 aerobic and anaerobic, and grows most readily in a medium slightly alkaline to 

 phenolphthalein. Besides serum or blood, glycerin, nutrose and dextrose are found to 

 be favorable for its growth. On agar it grows in small, rather transparent, finely 

 granular colonies, which are larger and more opaque when serum or ascitic fluid is 

 present. Broth is faintly and uniformly clouded. Milk is a favorable medium for 

 most strains and typically is acidified and coagulated. On potato, growth may occur 

 but is invisible. Gelatin can rarely be used at a temperature high enough to allow 

 growth. When occasionally growth is obtained the medium is not liquefied. On 

 blood serum, growth appears as small clear colonies and on the whole is better than on 

 agar. A number of special media are described of which one of the most valuable is the 

 inulin-serum-water medium of Hiss. It typically ferments, with the production of 

 acid, the majority of carbohydrates, even polysaccharides as inulin. On blood agar 

 the typical organism produces a greenish zone in the medium about the growth, but 

 not a clear zone of haemolysis as do most strains of streptococci. The differentiation 

 from other streptococci is sometimes a matter of difficulty, and the following characters 

 are of importance the lanceolate shape, capsule formation, fermentation of inulin, 

 absence of haemolytic powers, agglutination in antipneumococcic sera, susceptibility 

 to lysis by the action of bile salts. Acid is an important and characteristic product 

 and, if allowed to accumulate, rapidly kills the organism. The toxic products appear 

 to be closely united with the cell bodies and are only released when these are broken up. 

 The resistance to heat is not great and its thermal death-point is 52. Light is 

 germicidal if the cocci are not protected in thick masses of sputum. Drying is resisted 

 rather well in sputum or the blood of infected animals. To germicides the Pneumo- 

 coccus is very sensitive and is killed in a few minutes by the common disinfectants in 

 their usual strength. 



The pathogenic properties of the Pneumococcus for animals is some- 

 what variable. Natural infection is not common. To artificial infection 

 mice and rabbits have been found most susceptible, while guinea pigs, 

 dogs, rats and cats are more resistant, and birds are practically immune 

 probably because of their high body temperature. Mice and rabbits 

 succumb to subcutaneous or intraperitoneal injections of virulent cocci 

 from cultures or in sputum with the development of a septicaemic con- 

 dition, and in the latter case a peritonitis. By special methods lobar 

 pneumonia has been produced in rabbits as has also endocarditis. 



Variations in virulence of the Pneumococcus are very marked. The 

 virulence can be increased by passage through susceptible animals until 

 an extremely small dose will kill a mouse. Cultures obtained from man 

 may vary considerably in their virulence for animals. 



