282 CHEN CHONG CHEN AND LEO F. RETTGER 



tained in cultures which are only ten to fourteen hours old (chart 

 VI, upper part). 



THE DEPORTMENT OF MIXED CULTURES (COLI AND AEROGENES 



TYPES) TOWARD THE METHYL RED TEST AND THE 



VOGES AND PROSKAUER REACTION 



One strain of typical B. coli of fecal origin was grown in sym- 

 biosis with 56 different strains of B. aerogenes obtained from 

 soil. Each of the strains had been cultured separately in plain 

 broth for twenty-four hours, and a loopful of each tube was 

 inoculated into the Witte peptone- and the synthetic medium 

 of Clark and Lubs. The mixed cultures were incubated for five 

 days, one set at 24° and a second at 37°C. The hydrogen ion 

 concentration was then determined with methyl red as indicator, 

 and the Voges and Proskauer test was made as described earlier 

 in this paper. 



The results of the study are given in the accompanying chart 

 (VII) . They show that in the peptone-phosphate-glucose medium, 

 both at 24° and at 37°, the hydrogen ion concentration of the 

 aerogenes type of organism was partly disturbed, and that in 

 the synthetic medium the acidity of the colon type almost entirely 

 obscured that of B. aerogenes. The Voges and Proskauer reaction 

 as ordinarily given by the aerogenes type was not at all affected in 

 the synthetic medium at 24°, and only one culture in the peptone 

 medium failed to give the characteristic reaction when grown at 

 24°C. Of the 56 tubes incubated at 37°, 14 were negative by 

 the V and P test, 4 in the synthetic and 10 in the peptone medium. 



Briefly stated, when both types of organisms are present in 

 any given culture the methyl red test reveals the presence of the 

 colon type, while the aerogenes type responds to the Voges and 

 Proskauer test when the temperature of incubation has been as 

 low as 24°. At the higher temperature only a small number of 

 the cultures are negative to the V & P test. The permanency of 

 the Voges and Proskauer reaction is here clearly demonstrated. 



