BACILLI OF THE COLON-TYPHOID-DYSENTERY GROUP 629 



generation after isolation from the body, may be extremely motile, 

 while old laboratory strains may show almost no motility. Independent 

 of these modifying conditions, however, separate races may show indi- 

 vidual characteristics as to motilit\ r , varying in range between a motility 

 hardly distinguishable from Brownian movement and one which is so 

 active as to be but little less than that of the typhoid bacillus. Ordi- 

 narily, the colon bacillus possesses a motility intermediate between 

 these two extremes. 



FIG. 65. BACCILLUS COLI COMMUNIS. 



Cultivation. The bacillus is an aerobe capable of anaerobic 

 growth under suitable cultural conditions. It grows well on the sim- 

 plest media at temperatures ranging from 20 to 40 C., but finds its 

 optimum growth at about 37.5 C. Upon broth it grows rapidly, giving 

 rise to general clouding; later to a pellicle and a light, slightly slimy 

 sediment. Within moderate ranges, it is not delicately susceptible to 

 reaction, growing equally well on media slightly acid and on those of a 

 moderate alkalinity. 



Upon agar, it forms grayish colonies which become visible within 

 twelve to eighteen hours, gradually becoming more and more opaque 

 as they grow older. The deep colonies are dense, evenly granular, oval, 

 or round. Surface colonies often show a characteristic grape-leaf 



