COLON-AEROGENES GROUP OF BACTERIA 267 



EXPERIMENTAL 



In the present investigation the methyl red test has been 

 placed on a more strictly quantitative basis, and exact pH values 

 have been given to the various shades of color that were obtained 

 at different times, both in the "neutral tint" range as well as 

 where the characteristic light rose to deep red colors occurred. 



In view of the fact that the inconsistent results obtained by 

 some investigators were attributed by some to the indiscriminate 

 use of different brands of peptone, two well-known products 

 were employed, namely Witte's and Difco. The composition 

 of the medium was the same as that first recommended by Clark 

 and Lubs (0.5 per cent each of di-potassium phosphate, peptone 

 and glucose). The new synthetic medium of Clark and Lubs 

 was also used, for comparison. 



No definite standards as to the temperature and incubation 

 time for the methyl red test seems thus far to have been agreed 

 upon by different workers. Clark and Lubs chose 30°C. and an 

 incubation period of three to five days, whereas Levine main- 

 tained that no difference could be observed whether the temper- 

 ature was 30 or 37°C. He held that 3 days' incubation is suffi- 

 ciently long if the temperature is 37°C. More recently, Johnson 

 and Levine (1916) resorted to forty-eight hours' incubation at 

 37°; Burton and Rettger (1917), five days at 37°; and Winslow 

 and Cohen (1918), four days at 37°C. 



Preliminary studies showed that a large number of the soil 

 organisms refused, at least for a while, to grow at 37°C, especially 

 in the synthetic medium, thus apparently confirming the findings 

 of Rogers and his co-workers, and those of Winslow and Cohen. 

 Throughout the present study the incubation temperature was 

 30°C. Since there was still a question as to the most desirable 

 incubation period to be employed in order to obtain the most 

 uniform and dependable results, both a three and a five day 

 period were given a lengthy trial with all three of the media. 



The synthetic medium had a distinct advantage over the 

 others in being practically if not entirely colorless after sterili- 

 zation, and hence not obscuring the color reactions. However, 



