8 



TABLE I. LIQUEFACTION OF GELATIN BY 202 COLON STRAINS FROM SOIL. 



The long incubation period necessary makes this test an inconvenient 

 one for practical work and in some laboratories a modification has been 

 introduced employing 37 C. for 1 48 hours. This of course liquefies 

 the gelatin and so to determine whether a physiological decomposition 

 has taken place the tubes are immersed in ice water until control tubes 

 solidify. Inoculated tubes which remain liquid are regarded as having 

 been peptonized by the action of the organism in question. 



Indol. The production of indol from peptone is very extensively 

 determined particularly in England where indol formers are regarded as 

 "typical" act. coli. The test is usually carried out in the following man- 



ner : 



One percent peptone water cultures are incubated for four or five days 

 at body temperature. The culture is acidified with 1 c. c. of a ten percent 

 solution of sulphuric acid and then 1 c. c. of a 1-5,000 potassium nitrite 

 is added so as to form a layer on the surface. After a period varying from 

 a few minutes to an hour a red ring will develop at the junction of the 

 nitrite and acidified peptone culture if indol is present. This is known 

 as the Salkowfsky test. 



A more delicate reaction is obtained by the Ehrlich test which is per- 

 formed thus: 



To the culture add 3 c. c. para-dimethyl-amido-benzaldehyde and 3 

 c. c. of a saturated solution of potassium persulphate. Presence of indol 

 is indicated by the production of a red coloration. 



The significance and value of the test has been much in dispute. Howe 

 found it to be but slightly correlated with other characteristics and con- 

 sequently regarded it to be of little diagnostic value. Castellani and 

 Chalmers on the other hand, consider indol of fundamental importance in 

 classification of colon-like forms, and Houston believes it to be of par- 

 ticular significance for distinguishing the "typical" or "excretal" from 

 the "atypical" Bact. coli. Levine found that among members of the colon 

 group which were of intestinal origin, 91 percent formed indol, whereas of 

 those obtained from soil only 37.3 percent were indol positive. 



The constancy of the reaction has been questioned. Smirnow reported 

 that subjection of Bact. coli. to the action of carbolic acid induces a loss 

 of the ability to produce indol but that this character is regained after 

 several sub-cultures in nutrient broth. 



