390 



PATHOGENIC MICROORGANISMS 



(ration, 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 individual 

 characteristics as to motility, 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. 



Cultivation. The bacillus is an aerobe capable of anaerobic growth 

 under suitable cultural conditions. It grows well on the simplest m^dia 





m 2Vt^<%Jv%, 



FIG. 83. BACILLUS COLI COMMUNIS. 



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. 



