GENUS BACILLUS 311 



in the same culture. The bacilli occur singly or in pairs joined 

 end-to-end, and more rarely in short chains. Under certain 

 conditions the stained bacilli exhibit bipolar granules. Upon 

 some varieties seven or eight peritrichic flagella have been 

 demonstrated. 7 



Staining. The colon bacillus stains readily with the 

 ordinary aniline colors. It is always decolorized by Gram's 

 method. 



Cultivation. The colon bacillus develops on all the 

 usual culture media. Its growth is usually more abundant 

 than that of B. suipestifer or the typhoid bacillus but the 

 difference is not sufficient for -a differential diagnosis. Al- 

 though it grows best aerobically, yet it grows well anaero- 

 bically, especially in media containing sugars. 



Agar. The surface colonies are mostly circular, finely 

 granular, and rather opaque. The surface is glistening. The 

 deep colonies are apt to have protuberances. In streak cul- 

 tures an abundant, soft, white layer is quickly developed, but 

 the growth is not characteristic. 



Gelatin. On gelatin, colonies develop in from 18 to 36 

 hours. When located in the depths of the gelatin and ex- 

 amined by a low- power lens they are at first seen to be finely 

 granular, almost homogeneous, and of a pale yellowish to 

 brownish color; later they become larger, denser, darker, and 

 more coarsely granular. In shape they may be round, oval, 

 or whetstone shape. The superficial colonies appear as small, 

 dry, irregular, flat, blue- white points, that are commonly some- 

 what dentated at the margin. When the gelatin is not firm 

 the margins of many colonies are broken by outgrowths, which 

 are rather characteristic of colon bacilli. In stab cultures on 

 gelatin the growth usually takes the form of a nail with a 

 flattened head, the surface extension generally reaching out 

 rapidly to the sides of the tube. Savage 8 points out consid- 

 erable variation in the surface colonies of B. coli on gelatin. 

 He considers the variations to be influenced largely by the ma- 



7 Moore. The Wilder Quarter Century Book, p. 339. 



8 Savage. Jour, of Path, and Bact, Vol. IX (1903) p. 347. 



