io6 THE ACTION OF ENZYMES 



mould. The gelatine stab resembles a miniature inverted spruce 

 fir tree. Bouillon is rendered slightly turbid and a small amount 

 of indole is produced. Milk is not coagulated, and on potato 

 the growth is usually very slight. 



3. Bacterium coli, Escherich. Another organism which is 

 usually present in decaying flesh and other organic substances is 

 Bact. coli commune^ or Bact. coli (Esch.) (Fig. 26). . It is a con- 

 stant inhabitant in the intestines of all mammals, and has 

 been isolated from the digestive tract of birds and fishes. The 

 excreta of animals contain it in enormous numbers, and it may 

 be considered as one of the most widely distributed of all 

 bacteria. It is not infrequently met 

 with in milk and other dairy products 

 which have been contaminated with the 

 dung of cattle, and from manured soils 

 and those containing decaying organic 

 debris it finds its way into well and river 

 water. A very large number of allied 

 varieties and forms are known which 

 differ so very slightly from each other 

 FIG. 26. Bacterium coli, Esch: that their description and classification 



is a matter of great difficulty. 



Typical Bact. coli is a short, stumpy bacterium with rounded 

 ends, motile when young, and does not form spores nor stain by 

 Gram's method. It is generally single or united in pairs, each 

 cell being about 1.3 to 3/0, long, and .4 to .5 p broad It grows 

 best as an aerobe at 37 C, but will thrive under anaerobic 

 conditions, especially when cultivated in media containing sugar. 

 Gelatine. The surface colonies on gelatine are moist, trans- 

 lucent, whitish, flat and thin, with an irregular wavy outline, the 

 deep ones being round and yellowish-brown : there is no 

 liquefaction of the medium. In gelatine stab it forms a thick 

 white opaque growth along the track of the needle. 



Agar. On agar the colonies are smooth, shining, greyish- 

 white, elevated and not so irregular in outline as on gelatine. 



