516 PATHOGENIC MICROORGANISMS 



temperature for its development being 37.5° C. Temperatures above 

 37.5°, while not entirely stopping its growth, impede the development 

 of its toxin. 



Resistance. — ^The thermal death point of this organism is 58° C. for 

 ten minutes, according to Welch and Abbott. Boiling kills it in about 

 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' has found it 

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

 easily killed by chemical disinfectants in the strengths customarily 

 employed. H2O2 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 

 extract, 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 ^ 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.^ 



LoefjUer'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 LoefHer's serum. 

 Organisms 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, 



' Sternberg, " Manual Bag./' p, 455. 2 Michel, Cent. f. Bakt., 1897. 



