6l2 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



common is small grayish-white, rounded, slightly raised, almost translucent with 

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

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

 more luxuriant, white rounded, raised, granular to nearly smooth and somewhat moist. 

 Plain broth must be slightly alkaline to litmus. About one half of cultures grow 

 readily and half very feebly. The characteristic growth is a purely 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. Gelatin-growth is scanty 

 chiefly owing to temperature at which this medium must be kept. The gelatin is not 

 liquefied. Milk grows readily at comparatively low temperature (20) without coag- 

 ulation but with acid production. 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 Loeffler'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 2 52 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 of the host concerned in diphtheria is probably 

 different at the beginning from that involved late in the disease. Ex- 

 perimentally, agglutinins, bacteriolysins and opsonins have been dem- 

 onstrated in exudates and serum. While these properties may be im- 

 portant in warding off an infection they appear to be of little influence 

 once the bacteria are established and thereafter depending on the amount 

 of antitoxin will rest the outcome of the disease. 



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

 have not been demonstrated. 



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

 Direct infection by coughing, sneezing and speaking probably takes 



