64 MARGARET C. PERRY AND W. F. MONFORT 



DISCUSSION OF RESULTS 



It is apparent that in our results, as in instances earlier cited, 

 the methyl red and Voges-Proskauer reactions of the same 

 strain are not always of opposite signs. Variation in each, but 

 principally in the former, has been observed in the standard 

 broth. Ayers and Rupp (1918) have shown that reversion of 

 acid reaction exhibited by Bad. aerogenes cultures is due to the 

 secondary decomposition of organic acids and is accompanied by 

 rapid destruction of formic, acetic and other acids. With Bad. 

 coll they noted that acid formation does not run parallel with 

 the destruction of glucose, formic acid remaining constant or 

 being slightly reduced during the later stages of fermentation. 

 The distinction between Bad. coli and Bad. aerogenes is con- 

 sidered as lying chiefly in the difference in rate between the pre- 

 liminary decomposition of sugars into acids and the secondary 

 decomposition of the acids themselves. The suggestion was 

 made by Burton and Rettger that there may be a difference in 

 the rate of the secondary decomposition even in strains of Bad. 

 aerogenes which would explain divergence in the atypical strains 

 such as we are considering. They found the Voges-Proskauer 

 reaction more reliable than the methyl red reaction, as cited 

 above. The simpler uric acid reaction may prove even more 

 dependable than the complex A^oges-Proskauer reaction, being 

 accomplished in briefer period and permitting less modification 

 of the strain under cultivation. It is true that Rettger and 

 Chen found it possible to shorten the incubation period from 

 five days to twenty-four hours (even ten to fourteen hours) with- 

 out altering the Voges-Proskauer reaction; they also report the 

 successful use of Difco peptone in this test. But even with this 

 reduction in the time element the possibility of variation in the 

 complex reaction is not removed. 



The positive uric acid reaction overlaps the acid methyl red 

 and the negative Voges-Proskauer reactions in many strains. 

 If the observation is confirmed, that the Koser reaction gives 

 satisfactory correlation with the other reactions except among 

 colon strains from soils, this may prove of value in clearing up 



