272 CHEN CHONG CHEN AND LEO F. RETTGER 



Chart V is plotted with two curves representing 173 coli strains 

 isolated from animal feces. These are the results of the final 

 hydrogen ion concentration determinations in the synthetic and 

 the Witte peptone media after five days at 30°. Again the coli 

 cultures gave uniform results with the methyl red tests. The 

 pH values of these cultures cover a range of from 4.5 to 5.6 in 

 the Witte peptone, and 4.6 to 5.8 in the synthetic medium. 



Taking the three coli charts (III, IV and V) together, we find 

 no qualitative differences in the results obtained with the three 

 media or the two incubation periods, in so far as the methyl red 

 test is concerned. This goes to show that incubation for three 

 days at 30° C. is sufficient to give the characteristic methyl red 

 reaction by coli strains from both fecal and non-fecal sources. 



A careful analysis of these curves reveals several interesting 

 facts. We find that the hydrogen ion concentration of the aero- 

 genes cultures is generally higher in the synthetic than in the 

 Witte peptone-glucose-phosphate medium either after a three 

 or five days' incubation period. This would indicate that the pro- 

 duction of alkalinity by aerogenes strains is more rapid in the Witte 

 peptone medium than in the synthetic. In the former the peak of 

 the H. I. C. curve for the three days' period is in the majority of cul- 

 tures located at pH 6.3, while in the synthetic medium the peak 

 lies at 5.7. A still more striking difference is shown in the five-day 

 curves. In the synthetic medium the pH peak lies at 6.5, and 

 the curve covers a range of only 6.0 to 6.8, whereas in the Witte 

 peptone phosphate glucose medium the curve extends over a 

 wider pH scale, 6.0 to 7.6, with the peak at 6.8. 



In the Difco peptone medium the results are of peculiar interest. 

 Setting aside for the present the few discrepancies (26 out of 447 

 aerogenes cultures which gave the methyl red positive test), a 

 remarkable similarity between the Witte and the Difco peptone 

 medium curves is observed. They almost parallel each other, 

 from the range of pH 6.0 to 7.5. Their four peaks at pH 6.4, 

 6.6, 6.8 and 7.3, and the three troughs at 6.5, 6.7, and 7.1 corre- 

 spond almost exactly. 



