DIPHTHERIA. 209 



visible " moderate-sized free masses in which the slender, rather 

 long, slightly curved bacilli lie in characteristic irregular ar- 

 rangement an appearance that is to some degree represented 

 by placing the extended fingers of one hand in varying combi- 

 nations over or by the side of those of the other." 



Cultural Properties. The diphtheria-bacillus thrives only 

 at temperatures between 20 C. (68 F.) and 40 C. (104 F.) 

 upon all slightly alkaline culture-media. Its temperature-opti- 

 mum is that of the body. 



Upon gelatin-plates round, whitish colonies form that remain 

 small. Microscopically these appear yellowish brown and gran- 

 ular, with an irregular border. 



In gelatin stab-cultures similar small, whitish, spherical colo- 

 nies, not exceeding a certain size, form along the line of inocu- 

 lation. At a temperature of 24 C. (75.2 F. ) superficial 

 growth takes place, with indications of nail-culture. The gela- 

 tin is not liquefied. 



Upon agar, and better upon glycerin-agar plates, after from 

 twenty-four to forty-eight hours, small, grayish-white, glistening 

 colonies form that often exhibit microscopically concentric lami- 

 nation ; with low powers of the microscope they appear pecu- 

 liarly granular, with an irregular border. 



In agar streak- cultures, after twenty-four hours, small, trans- 

 lucent, slightly raised colonies appear. Further growth is in- 

 considerable, and scarcely extends beyond the line of inocula- 

 tion. 



In agar stab -cultures the colonies develop along the line of 

 inoculation, and there is slight growth upon the surface. 



Upon Loffler s serum, after twenty-four hours, fairly large, 

 whitish, opaque colonies of firm consistence appear. In the 

 succeeding days these colonies increase but little in size. 

 Loffler's blood-serum (p. 84) constitutes, all in all, the best 

 culture-medium for diphtheria-bacilli. It is, therefore, always 

 employed for purposes of differential diagnosis. 



\Kbouillon, after twenty-four hours some precipitate has formed, 

 after two days slight turbidity, which increases to the fifth day, 

 then to grow less, until finally the fluid overlying the crumbling, 

 flocculent precipitate is completely clear. Not rarely a thin, 

 fragile coating appears upon the surface of the bouillon. 



A delicate coating forms upon the surface of potato rendered 

 alkaline. 



Milk constitutes a favorable nutritive medium, but is not 

 coagulated. 



In boiled and unboiled egg, both white as well as yolk, the 

 diphtheria-bacillus grows well ; upon coagulated egg-albumin 

 it not rarely exhibits branching. 



In slightly alkaline Loffler's meat-infusion bouillon the diph- 



