COLON-AEROGENES GROUP OF BACTERIA 



269 



border or methyl red line to separate the coli from the aerogenes 

 types of organisms. Since the colon type is now accepted as 

 methyl red positive, and since the pH value 6.0 is the end point 

 of the methyl red range, this line which starts at pH 6.0 appears 

 to be the natural line of demarcation between these two types, and 

 to be of value in plotting the charts. 



Chart I gives the results of the hydrogen ion concentration 

 determinations of the 447 aerogenes strains grown in synthetic 

 and Witte's peptone media for three days at 30°C. Both curves 



Chart 1. Frequency Curves for B. aerogenes Type (Soil) 



Ordinate represents numbers of strains and abscissa the pH values. Three 

 days' incubation at 30°C. Unbroken line, synthetic medium; broken line, 

 peptone-glucose-phosphate medium (both of Clark and Lubs). 



show clearly that a three day period of incubation is not sufficient 

 at this temperature for the methyl red test, especially in the 

 synthetic medium, in which almost all aerogenes cultures become 

 .methyl red positive. The peak of the curve for the synthetic 

 medium is located at pH 5.7 and even with the dark medium, a 

 large number of aerogenes cultures enter the methyl red range, 

 although not as many, as in the synthetic medium. 



Chart II shows the results of five days' incubation in the three 

 media. There is a striking difference between the results of the 



