frequently made, that the group consists of aerobes which ferment lactose 

 is somewhat confusing, for the characteristic fermentation of lactose (gas 

 formation) is determined under anaerobic conditions. 



DIFFERENTIAL TESTS 



The colon group, as defined, is a large and complex one including a 

 number of closely related but distinct species and varieties. The question 

 arises whether all the members are of equal sanitary significance or whether 

 some may not be more intimately associated with animal or particularly with 

 human pollution thereby becoming of special significance in the interpreta- 

 tion of water or other examinations. It may be of practical value to dis- 

 tinguish and classify these different types and, if possible, to correlate them 

 with their habitat. For these purposes a large number of tests have been 

 employed. 



The more important reactions to be considered are: 



Coagulation of milk. 



Gelatin liquefaction. 



Production of indol. 



Motility. 



Fermentation of carbohydrates. 



Production of acetyl-methyl-carbinol . (Voges Proskauer reac- 

 tion ) . 



Uric acid test. 



Methyl red test. 



Coagulation of milk. The test for the determination of the co- 

 agulation of milk is made by inoculating litmus or brom cresol purple 

 milk which is then incubated at the body temperature for 48 hours. Acid 

 is formed, some gas develops, and coagulation usually takes place in this 

 time. If the milk has not been clotted the tube is immersed in boiling 

 water for a few minutes or brought to a boil over a flame and if this treat- 

 ment induces coagulation the reaction is considered as positive. If the 

 milk has been over sterilized in the autoclave, coagulation does not take 

 place as readily as when the medium is sterilized by the intermittent method 

 in the Arnold. There is no digestion of the curd and the whey, if present, 

 is clear. The litmus may be reduced by some strains. The milk reaction 

 has been found very valuable in identification of the colon group. 



Gelatin liquefaction. The liquefaction of gelatin is an important 

 test for the differentiation of colon species. Studies by Gligler and by 

 Johnson and Levine indicate that this test is well correlated with motility 

 and fermentation of glycerol. Unfortunately the liquefaction of gelatin is 

 difficult to determine. The period of incubation usually employed is 

 fourteen days at 20 degrees C. Gage and Phelps, and later Johnson and 

 Levine pointed out that the proportion of liquefiers recognized varies with 

 the period of incubation as mav be seen from Table I. 



Among 202 strains studied by Johnson and Levine, 106 (52%) were 

 gelatin liquefiers after 34 days incubation but only 31 (15.3%) showed 

 this reaction in 13 days. 



