MICROBIAL DISEASES OF MAN AND DOMESTIC ANIMALS 763 



- more or less granular surface and dark center, the margins varying in irregularity, 

 and often with a thin extension spreading out from the edge; (6) less common, 

 larger, more luxuriant, white, rounded, raised, granular to nearly smooth and some- 

 what moist. Plain broth must be shghtly alkaline to litmus. About one-half of 

 cultures grow readily and half very feebly. The characteristic growth is a finely 

 granular deposit at bottom and along sides of the tube leaving the broth clear; a 

 few cultures produce a diffuse cloudiness with more or less well-developed pellicle. 

 Growth on geladn is scanty chiefly owing to temperature at which this medium 

 must be kept. The gelatin is not liquefied. In milk growth takes place at com- 

 paratively low temperature (20°) without coagulation but with acid production. 

 On potato growth occurs especially if slightly alkaline; in the majority of cases 

 invisible; it may appear as a thin dry glaze or with a whitish or slightly yellowish 

 tinge. On LoeflSer's blood serum growth is rapid. In eighteen to twenty-four 

 hours colonies are rounded, grayish- white with a slightly yellowish tinge moderately 

 translucent except toward the center, smooth, moist and shiny. The margins are 

 only slightly irregular. With age the colonies become dull and opaque, the surface 

 becoming marked by concentric lines and sometimes also exhibiting radial striation. 

 Thermal death-points are 58° to 60° for ten minutes, 70° for five minutes, 100° 

 for one minute. On the other hand —190° for seven days and —252° for ten 

 hours have failed to kill in some instances. Diffuse light hinders growth. Direct 

 sunlight kills within two hours to a few days according to the medium in which the 

 organisms are suspended. 



The organism gains entrance into the body through the mouth and 

 nose. 



The bacteria usually remain localized; they can almost always be 

 demonstrated when a definite membrane is present and often when: 

 there is none. They are practically always found in the lung of fatal 

 cases because of direct infection. Entrance of the bacteria into the 

 blood stream with resulting infection of the internal organs has occurred 

 in fatal cases. 



The protective apparatus concerned in diphtheria is probably dif- 

 ferent at the beginning from that involved late in the disease. Experi- 

 mentally, agglutinins, bacteriolysins and opsonins have been demon- 

 strated in exudates and serum. While these properties may be 

 important in warding off an infection they appear to be of little influ- 

 ence once the bacteria are established, and thereafter on the amount 

 of antitoxin will rest the outcome of the disease. 



The toxin is strongly antigenic, the cell bodies feebly so. Aggressins 

 havje not been demonstrated. 



.The organisms escape by the secretions of the mouth and nose. 

 Pirect infection by coughing, sneezing and speaking probably takes 



