COLON-AEROGENES GROUP OF BACTERIA 



261 



Gelatin liquefaction 



ORGANISMS TESTED 



B. aerogenes from soil 



B. coli from soil 



B. coli from feces 



NUMBER 

 EXAMINED 



447 



20 



173 



The tests for indol were made by the Salkowski and by the 

 para-dimethyl-amido-benzaldehyde method of Ehrlich (1901). 

 In the former procedure 0.5 cc. of a 10 per cent solution of sul- 

 phuric acid and 0.5 cc. of a 0.01 per cent solution of potassium 

 nitrite were added to 5 cc. of a five-day old culture which had 

 been incubated at 30°C. The Ehrlich test was conducted by 

 adding to the same amount of culture material 0.5 cc. of a 2 per 

 cent solution of para-dimethyl-amido-benzaldehyde in 95 per 

 cent alcohol and then concentrated hydrochloric acid drop by 

 drop and in such a way as to keep the two layers of fluid separate. 

 The medium employed in both tests consisted of 10 grams of 

 Witte's peptone, 5 grams of sodium chloride, 0.2 grams of dipotas- 

 sium phosphate, and 1000 cc. of water. The results of these 

 tests are given in the following table. 



Indol production- 



ORGANISMS 



B. aerogenes from soil 



B. coli from feces 



B. coli from soil 



NUMBER OP 

 STRAINS 

 TESTED 



447 

 173 

 20 



The marked difference between the results obtained by the 

 s two methods with the aerogenes strains from soil is noteworthy, 

 and is due either to the extreme sensitiveness of the Ehrlich 

 reagent, or to the fact that the Salkowski method is in reality 

 not an indicator of indol, but of some other reacting substance, 

 as has been claimed by Kligler (1914) and others. All of the 

 fecal strains of B. coli were indol-positive. 



