276 CHEN CHONG CHEN AND LEO F. RETTGER 



The interpretation of the results is fraught with some difficulty. 

 Clark and Lubs, and Ayers and Rupp did not deny that ammonia 

 production may accompany the phenomenon of reversion. Ken- 

 dall's theory of ammonia recession may in part explain a certain 

 phase of reversion, and the claim of Rettger and Berman, and 

 others, that ammonia is a readily available food for bacteria, 

 and that it is not an end product, but an intermediary stage 

 of metabolism, may throw considerable light on the subject. 



GAS PRODUCTION 



Escherich (1885) clearly demonstrated the production of gas 

 in glucose and lactose media by his two types of intestinal organ- 

 isms. Arloing confirmed the observation of Escherich. It was 

 not until after the appearance of the fermentation tube (1890- 

 93) of Theobald Smith, however, that the determination of gas 

 volume and ratio formed part of our scheme of differentiation 

 of bacteria. For almost twenty years this method reigned 

 supreme and its accuracy as a quantitative procedure was little 

 questioned. 



Although Harden, Walpole and Thompson had improved the 

 Smith method, the analytical error was not eliminated. Keyes 

 (1909) introduced the vacuum method of exact gas analysis, 

 and determined the gas ratio of B. coli. His work was followed 

 by that of Rogers and his associates (1914-15), who firmly estab- 

 lished the gas ratio of B. coli and B. aerogenes, namely approxi- 

 mately 1: 1 (C0 2 :H) for the former, and 2: 1 for the aerogenes 

 type. Since then there appears to be no doubt that the principle 

 of gas-ratio as a differential test is well founded. 



In the present study the gas production was at first observed 

 in the Durham fermentation tube. A gas volume of at least 

 10 per cent was recorded as a positive test for gas, while the acid 

 reaction in the same tubes was roughly determined with Brom- 

 cresol-purple, a distinct yellow color being recorded as a positive 

 reaction. In the use of the Durham fermentation tube a long 

 inner tube (3 inches) was employed to facilitate the reading of 

 the gas volume, which was done by the Frost gasometer. Al- 



