17 



II. EVIDENCE OF TWO SUBDIVISIONS IN THE COLON GROUP 

 AND TESTS FOR THEIR DIFFERENTIATION. 



The mass of recent work clearly demonstrates that the colon group 

 includes two quite distinct subgroups which differ culturally but partic- 

 ularly with respect to carbohydrate and nitrogen metabolism. These sub- 

 divisions, which will be referred to hereafter as the coli and aerogenes 

 sections, are also quite strikingly correlated with habitat, the former pre- 

 dominating in feces and sewage, the latter in the soil and on grains. The 

 evidence for this subdivision together with a detailed consideration of the 

 differential reactions employed is presented here. 



General Differential Characteristics. Escherich, in 1885, distin- 

 guished Bact. coli commune from Bact. (lactis) aerogenes by the greater 

 plumpness of the latter, its lack of motility, and its more rapid coagulation 

 of milk. Later Smith, in 1893 and 1895, indicated that the Bact. aerogenes 

 produced a heavier growth and showed a tendency toward capsule forma- 

 tion. He also remarked that gas production was more rapid from glucose 

 and that the proportion of carbon dioxide to hydrogen was greater. Chen 

 and Rettger observe that in glucose broth the volume of gas is seldom 

 greater than 40 percent with the coli section, whereas the aerogense sub- 

 group frequently produced much more gas. 



TABLE VI. GAS PRODUCTION IN 1% GLUCOSE BROTH 

 (After Chen and Rettger, 1920) 



Burton and Rettger report also that in a modified Uschinsky medium 

 (glucose substituted for glycerol) the coli subgroup grows very poorly, if 

 at all, whereas the aerogenes strains show vigorous growth with almost 

 complete utilization of the sugar. 



Temperature Relationships. In some very careful studies on the 

 rate of multiplication of Bact. coli, Barber found that the rate increased 

 with increase in temperature showing a maximum at 44 degrees to 45 de- 

 grees C. A temperature of 40 degrees C. has often been recommended for 

 the isolation of the colon group from water and in the Eijkman test 46 

 degrees C. is employed. 



With reference to the aerogenes section, we find that Rogers and his 

 associates often mention the necessity for using a relatively low temperature 

 (30 degrees C.) for growth of some strains isolated from grains. This 

 observation is concurred in by Winslow and Cohen, and more recently 

 Chen and Rettger report that in studies of soil organism a large number 



