264 CHEN CHONG CHEN AND LEO F. RETTGER 



ary alkali production sufficient in some cases entirely to obscure 

 the acid formation." They, furthermore, discredited the salicin 

 test on the ground that too large a percentage of their own cul- 

 tures (94.6 per cent) fermented this substance. 



Levine (1916) maintained that the origin of a strain correlates 

 better with sucrose than with the sucrose-dulcitol fermentation. 

 In his second paper (1916) he concluded that "quantitative acid 

 production in glucose, galactose, maltose, lactose, sucrose, raf- 

 finose, salicin, inulin, mannitol, dulcitol and glycerol is not a 

 reliable index for differentiating colon-bacillus-like bacteria." 

 He believes that gas formation is of more value in classification. 



Murray (1916) attempted to differentiate human, bovine and 

 equine types of colon bacilli by means of quantitative acid pro- 

 duction. He came to the same conclusions as Levine. "In all 

 cases," he says, "the average acid production for each of the 100 

 strains of each type resembles that of every other, and also 

 resembles the average acid production of all the strains taken 

 together irrespective of origin." 



Hulton (1916) studied 45 strains of colon bacilli from various 

 sources and found that a better correlation was obtained between 

 sucrose fermentation and source than between sucrose-dulcitol 

 fermentation and source, confirming Levine's observations. 



It appears very clear, from the above cited observations, that 

 acid production, even when determined by the biometric method, 

 does not furnish a sound basis for the classification of the colon- 

 aerogenes group of bacteria. The relationship between the 

 various cultural and physiological characters and the normal 

 habitat of the organisms studied has been in the past lamentably 

 neglected. This important phase of classification has recently 

 been brought to light, however, by Rogers (1914-1916) and his 

 associates. The grouping of the colon bacilli has by them been 

 greatly simplified and placed upon a more natural and logical 

 foundation. In their papers dealing with the bacteria isolated 

 from milk and milk products, feces and grains, they gave us a 

 sound criterion for the separation of the colon group into two 

 distinct types. This separation was accomplished by the deter- 

 mination of the gas ratio in a glucose medium. 



