392 ACTINOMYCETES. 



granules (60, x, xi). Nevertheless virulent diphtheria 

 bacilli without granules occur, although very rarely (see 

 Kurth, I. c), so that a lack of the granules does not 

 exclude the diagnosis of diphtheria. (See also p. 402. ) 



Relation to Oxygen. — Optimum growth with entrance 

 of air ; when oxygen is excluded, the growth is lessened. 



Requirements as Regards Temperature, Reaction, 

 and Composition of the Nutrient Medium. — It grows 

 well and abundantly at incubator temperature only. 

 Optimum temperature 33° to 37°; extremes, about 18°-20° 

 and 40°. Glycerin-agar is more favorable for its growth 

 than ordinary agar, but serum or ascitic nutrient medium 

 are much better. Loffler's blood-serum mixture is much 

 used (Technical Appendix) ; also Tochtermann's and 

 CDeyke's nutrient media are highly recommended (Techni- 

 cal Appendix). Since we have used glycerin-ascites-agar 

 almost exclusively instead of glycerin-agar, we have 

 obtained excellent results, but one must become accus- 

 tomed to the relatively luxuriant appearance of the growth. 

 Upon gelatin at 22°-24° the growth is so absolutely with- 

 out characteristics (no liquefaction), and so scanty, that 

 such cultures are never prepared. 



Gelatin Stab. — Along the stab canal only a slight 

 growth. The surface growth is yellowish-white, a little 

 elevated, with a smooth wavy border and in part lobulated. 

 It is faintly shining. 



Glycerin-agar Plates. — (a) Natural size: Circular or 

 roundish colonies, white to dirty yellow. The border is 

 smooth, they are more or less elevated, and with a moist 

 or faint luster. Many cultures present more luxuriant 

 (58, vn a) and many more delicate growths (58, vn b). 



(b) Magnified sixty times : The colonies present their char- 

 acteristic form after twenty-four hours at 37°. They are 

 small, roundish, usually exceedingly transparent colonies 

 of a grayish-yellow or brownish color. At the periphery 

 they are usually split or torn, and almost without excep- 

 tion are markedly crumbly. Many colonies appear at the 

 periphery as if raveled out. Still, according to the cul- 

 ture, they are thinner or thicker, lighter or darker, coarsely 

 or finely granular (59, i a and b). After two days the 

 colonies are thicker, somewhat irregular at the periphery, 



