628 PATHOGENIC MICROORGANISMS 



istics upon special media and by the- study of serum reactions in specific 

 immune sera. Our mainstays in the accurate differentiation of these 

 bacilli are their fermentative actions upon carbohydrate media, and 

 their agglutinating reactions in immune sera. These points will be 

 taken up in the description of the individual microorganisms, and will 

 again be summarized in the differential tables given at the end of the 

 chapters dealing with this group. 



BACILLUS COLI COMMUNIS AND MEMBERS OF THE COLON 

 BACILLUS GROUP 



Under the name of " colon bacilli" are grouped a number of varie- 

 ties differing from one another in minor characteristics, but cor- 

 responding in certain cardinal points which stamp them as close 

 relatives and amply warrant their consideration under one heading. 

 While usually growing as harmless parasites upon the animal and human 

 body, and capable of leading a purely saprophytic existence, they may, 

 nevertheless, under certain circumstances become pathogenic and thus, 

 both culturally and in their pathological significance, form a link between 

 pure saprophytes like Bacillus lactis aerogenes, on the one hand, and 

 the more strictly pathogenic Gram-negative bacilli of the paratyphoid, 

 typhoid, and dysentery groups, on the other. As a type of the group 

 we may consider in detail its most prominent and thoroughly studied 

 member, Bacillus coli communis. 



BACILLUS COLI COMMUNIS 



This microorganism was seen and described by Buchner 1 in 1885. 

 It was thoroughly studied in the years immediately following, by Esche- 

 rich, 2 in connection with the intestinal contents of infants. 



Morphology. Bacillus coil communis is a short, plump rod about 

 1-3 micra long, and varying in thickness from one-third to one-fifth 

 of its length. Under varying conditions of cultivation, it may appear 

 to be more slender than this or shorter and even coccoid in form. In 

 stained preparations, it usually appears singly, but occasionally may be 

 seen in short chains. It stains readily with the usual anilin dyes and 

 decolorizes by Gram's method. Spores are not formed. It is motile, 

 and flagella staining reveals eight or more flagella peripherally arranged. 

 Its motility is subject to wide variations. Young cultures, in the first 



1 Buchner, Arch. f. Hyg., 3, 1885. 



*E8cherich, "Die Darmbakt. des Sauglinngs, " Stuttgart, 1886; Cent. f. Bakt., 

 1, 1887. 



