290 CHEN CHONG CHEN AND LEO F. RETTGER 



(173) types, aside from the reactions in sucrose and adonitol, and 

 indol production of the aerogenes type. 



A complete study of gas production in the Durham fermenta- 

 tion tube was made of all of these organisms, both as to volume 

 and gas ratio, in so far as the method would permit, 40 per cent 

 being the arbitrary limit placed for the coli type, and above 40 

 per cent being taken as an indication of the aerogenes type. 

 Though very crude, this method permitted us to assume from 

 the results obtained a complete correlation between gas volume, 

 gas ratio, the methyl red, uric acid and the V and P reactions. 

 However, on account of the very faulty method employed in the 

 quantitative study of gas production these results are not included 

 in the charts or in any of the tables. 



GENERAL DISCUSSION 



A survey of the present investigation leads to the conclusion 

 that there exist in nature two distinct types within the colon 

 group of bacteria. These two types, now generally designated 

 as B. coli and B. aerogenes, can conveniently be set apart from 

 each other by the newer tests. 



The direct plating method is better adapted for the isolation 

 of the soil organisms than the combined preliminary enrichment 

 and subsequent plating procedure, because it permits of the study 

 of numerical relationships between bacterial types and their 

 habitat and of the determination of the relative numbers of each 

 type. 



The failure to isolate B. aerogenes from the feces of man and 

 animals in the present study should not be taken as evidence 

 that this organism is absent from the intestine, but rather as a 

 result of circumstances which permitted its being overlooked. 

 This apparent absence or scarcity is not at all surprising, as 

 Rogers and his associates (1914) found only one high ratio strain 

 in 150 coli-like organisms isolated from bovine feces. 



Nothing of importance could be gained through the morpho- 

 logical study of this group of bacteria, since both types present 

 practically the same microscopic picture. The indol test is of 



