144 PNEUMONIA 



The Bacillus. The pneumococcus, known also as Diplococcus 

 lanceolatus, and Diplococcus pneumoniae, is a small lanceolate organ- 

 ism, often with spherical and ovoid individuals. It presents marked 

 variations in shape and size, but as a rule the number of typical indi- 

 viduals renders indentification easy. It is non-motile and takes the 

 ordinary stains easily. It is a capsulated bacillus and when in diplo- 

 coccus forms, both individuals are enclosed by the same capsule. In 

 chain forms, not commonly seen, several links in the chain may be 

 covered by the same capsule. The formation of the capsule is depend- 

 ent on conditions. Highly virulent organisms taken from the animal 

 are capsulated. On the other hand, subcultures grown on artificial 

 media through many generations have no capsules or show imper- 

 fect ones. Growth in blood serum seems to favor capsular develop- 

 ment. It seems most probable that the formation of the capsules is 

 a protective function. The pneumococcus grows on ordinary culture 

 media, but not abundantly, and is highly susceptible to slight modi- 

 fications in composition and reaction. Fresh cultures do not develop 

 on gelatin below 24 C, but old subcultures grow at temperatures as 

 low as 20 C. There is no liquefaction. After twenty-four hours in 

 the incubator, bouillon tubes are slightly cloudy and may show a 

 flocculent deposit. As a result of the autolysis of the cells the bouillon 

 may partially clear up after some days. On account of the low spe- 

 cific gravity of the cells a high speed is necessary to completely deposit 

 the organism in a centrifuge. On potatoes there is often no growth, 

 sometimes a slight, scarcely visible coat. In milk this bacterium 

 develops with a scanty production of acid which may suffice to 

 coagulate. Media containing from 4 to 6 per cent, of glycerin and 

 those containing blood or serum furnish richer growths. Old sub- 

 cultures develop on artificial media more rapidly and abundantly 

 than fresh, virulent cultures. Air supply has no marked effect, one 

 way or the other, on the growth of this organism. It needs frequent 

 transplantation in order to preserve both viability and virulence. It 

 is easily destroyed by the ordinary disinfectants, both liquid and 

 gaseous. 



There is probably no other pathogenic bacterium of such diver- 

 gent virulence as this. I have worked with two strains, one of which 

 is fatal to guinea-pigs after intraperitoneal injection only in doses of 



