518 Bacilli Resembling the Typhoid Bacillus 



Morphology. The bacillus is rather variable, both size 

 and form depending to a certain extent upon the culture 

 medium on which it grows. It measures about 1-3 X 0.4- 

 0.7 fJL. It usually occurs in the form of short rods, but 

 coccus-like individuals and elongate individuals may be 

 found in the same culture. The bacilli are usually separate 

 from one another, though occasionally joined in pairs, are 

 actively motile, and provided with flagella, which are vari- 

 able in number, usually from four to a dozen. The organ- 

 isms from some cultures swim actively, even when the cul- 

 ture is some days old ; others are sluggish even when young 



Fig. 153. Bacillus coli communis; superficial colony two days old 

 upon a gelatin plate. X 21 (Heim). 



and actively growing, and still other cultures consist of 

 bacilli that scarcely move at all. It forms no endospores. 



Staining. The bacillus stains well with the aqueous 

 solutions of the anilin dyes, but not by Gram's method. 



Cultivation. It is readily cultivated upon the ordinary 

 media. 



Colonies. Upon gelatin plates the colonies are visible 

 in twenty-four hours. Those situated below the surface 

 appear round, yellow-brown, and homogeneous. As they 

 increase in size they become opaque. The superficial 

 colonies are larger and spread out upon the surface. The 

 edges are dentate and slightly resemble grape-vine leaves, 

 often showing radiating ridges suggestive of the veins of a 

 leaf. They may have a slightly concentric appearance. 



