r )lf) PATHOGENIC MICROORGANISMS 



one minute. Low temperatures, and even freezing, are well borne. 

 Desiccation and exposure to light are not so fatal to this organism as 

 to most of the other pathogenic bacteria. Sternberg 1 has found 

 alive in dried bits of the pseudomembrane after fourteen weeks. It is 

 easily killed by chemical disinfectants in the strengths customarily 

 employed. H 2 2 seems especially efficacious in killing the organisms 



rapidly. 



Cultivation. The diphtheria bacillus grows readily on most of the 

 richer laboratory media. It will grow upon media made of meat ex- 

 tract, but develops more luxuriantly on all those which have a meat 

 infusion as their basis. While it will grow upon both acid and alkaline 

 media, it is sensitive to the extremes of both, the most favorable reaction 

 for its development being probably about 0.5 per cent alkalinity ex- 

 pressed in terms of T NaOH. Animal proteids added to the media, 

 in the form of blood serum, ascitic fluid, or even whole blood, increase 

 greatly the rapidity and richness of its growth. Horse serum is sup- 

 posed by some to be especially favorable. 2 



Loeffler's Medium. The most widely used medium for the cultiva- 

 tion of this bacillus is the one devised by Loeffler. This consists of: 



Beef blood serum 3 parts 



One per cent glucose meat-infusion bouillon 1 part 



The mixture is coagulated at 70 C. in slanted tubes and sterilized at 

 low temperatures by the fractional method. Upon this medium the 

 diphtheria bacillus in twelve to twenty-four hours develops minute, 

 grayish-white, glistening colonies. These enlarge rapidly, soon out- 

 stripping the usually accompanying streptococci. The medium seems 

 to possess almost selective powers for the bacillus and, for this reason, 

 it is especially valuable for diagnostic purposes. 



Meat-Infusion Agar.- Upon slightly alkaline meat-infusion agar the 

 bacillus develops readily, though less so than on Loeffler's serum. Or- 

 ganisms which have been on artificial media for one or more genera- 

 tions may grow with speed and luxuriance upon this medium. When 

 planted directly from the human or animal body upon agar, however, 

 growth may occasionally be slow and extremely delicate. Colonies on 

 agar appear within twenty-four to thirty-six hours as small, rather trans- 

 lucent, grayish specks. The appearance of these colonies is quite char- 

 acteristic and easily recognized by the practiced observer. Surface colo- 



1 Sternberg, " Manual Bac.," p. 455. 2 Michel, Cent, f . Bakt., 1897. 



