380 A MANUAL OF BACTERIOLOGY 



nection with the Bacillus typhosus, in pathological pro- 

 cesses, and in water supplies as an indication of pollution. 

 As its name implies it is a constant inhabitant of the 

 intestinal tract in man and animals (except perhaps in 

 certain arctic animals), and is one of the most widely 

 distributed organisms in nature. While the term " colon 

 bacillus " is applied to a fairly well-defined organism (the 

 " typical B. coli "), there are a number of allied organisms 

 differing from the type in one or more characters e.g. 

 motility, indole production, fermentation reactions, rate and 

 extent of milk curdling, etc. and these varieties are said to 

 belong to the " colon group," or are termed " coliform." 



The B. coli may be readily isolated by inoculating litmus 

 lactose bile-salt peptone-water tubes with a trace of a 

 suspension of fresh faeces, growing for from twenty-four 

 to forty- eight hours at 42 C., and plating the culture on 

 litmus lactose agar, on gelatin, or on Conradi-Drigalski 

 agar, or by direct plating of the faeces suspension on the 

 last-named medium (see also " Water "). 



Morphology. The B. coli is a short rod with rounded 

 ends, 2 or 3 /x long and 0*5 /m broad, frequently linked in 

 pairs or more. It is often so short that it is merely ovoid 

 in shape ; and, on the other hand, longer individuals and in- 

 volution forms occur 10 /UL or more in length (Plate XIII. 6). 

 It is feebly motile, and possesses lateral flagella to the 

 number of three or four on an average, which are 

 shorter and straighter than those of the typhoid bacillus. 

 It is sometimes met with in diplococcoid form, which by 

 cultivation in ascitic fluid may become fixed. Capsulated 

 forms have been described. 



Spore-formation does not occur, but vacuolation may 

 sometimes be observed. The organism stains well by 

 the ordinary anilin dyes, but is Gram-negative. 



Cultural characters. The B. coli is aerobic and faculta- 

 tively anaerobic, and grows readily on the ordinary culture 



