158 PATHOGENIC BACTERIA 



yet been made. It can be said in general that all members 

 of this group find the intestinal tract a suitable place for life, 

 some under normal, others under pathological conditions. 

 Old classifications of the typhocolon group admitted only 

 organisms capable of motion, but some later observers 

 include many non-motile and even encapsulated forms. 

 Inasmuch as a very close separation on the basis of techni- 

 calities is not necessary in this work, it has been deemed 

 best to choose the principal clearly defined species for descrip- 

 tion. Such descriptions permit of extension in a general way 



FIG. 49. Colon bacilli. Twenty-four-hour agar culture. X 1100 

 diameters. (Park.) 



to the nearest congeners, and therefore we may say that we 

 are considering types. The typhoid and paratyphoid bacilli 

 have been sufficiently described in Chapter IX. 



The Colon Bacillus. The colon bacillus proper, called also 

 the Bacillus coli communis, is a non-spore-bearing, sluggishly 

 motile, delicate rod, measuring from 25000 to 8~oVo mcn m 

 length and 60 ioo to 30000 mcn in width. It appears when 

 stained as a single rod usually, but occasionally in pairs or 

 short chains. It takes the laboratory dyes with ease, usually 

 more deeply near its rounded ends. It moves by flagella 



