800 MICROBIOLOGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



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 dif- 

 ferentiation 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 antipneu- 

 mococcic sera, susceptibility to lysis by the action of bile salts. Acid is an im- 

 portant 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 pro- 

 tected in thick masses of sputum. Drying is resisted rather well in sputum or the 

 blood of infected animals. To germicides the Pneumococcus 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 infec- 

 tion 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 are 

 regularly used for the rapid isolation and determination of pneumo- 

 coccus types. 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. 

 In standardizing type sera, strains are used of which o.oooooi c.c. of a 

 broth culture will kill a mouse. 



The organism gains entrance through the respiratory mucosa and 

 as a matter of fact appears to be a common inhabitant of these regions. 

 However the organism may reach the lung (the lobar distribution sug- 

 gests sowing by the blood stream), it is certainly frequent to find posi- 

 tive blood cultures during the disease a fact which accounts for the 

 development of such complications as meningitis, endocarditis, etc. 



