VARIETIES OF COLON BACILLI 177 



made up of 4 parts of para-dimethyl-amido-benzalde- 

 hyde, 380 parts of absolute alcohol, and 80 parts of con- 

 centrated HC1; No. 2 is a saturated aqueous solution 

 of potassium persulphate ; 5 c.c. of i is added to 10 c.c. 

 of a broth culture and then 5 c.c. of 2 is added and the 

 whole shaken. A red color indicates indol. Mac- 

 Conkey (1909) who was at first inclined to discard indol 

 as a routine test, believes that when made in this way 

 it is of much value. 



Howe (1912), from a statistical study of 630 strains 

 of intestinal colon bacilli, concluded that indol, ammonia, 

 and nitrite tests were but slightly correlated with general 

 vigor and had but slight classificatory significance. It 

 does not necessarily follow, however, that this is true 

 of the forms which occur in stored waters. The so- 

 called " atypical B. coli " are of course rare in faeces but 

 they may occur in sufficient numbers to be important 

 in waters which have been remotely polluted. 



The liquefaction of gelatin is another test generally 

 applied in any detailed study of the colon group. The 

 longer the tubes are kept the higher will be the propor- 

 tion of positive results, for there are many slowly lique- 

 fying forms grading by almost imperceptible degrees 

 into the commoner non-liquefying type. The table 

 cited from Gage and Phelps (1903) on p. 186 shows that 

 of a series of 1908 cultures from various waters, sewages, 

 and shellfish 8 per cent liquefied gelatin in 4 days, 10 

 per cent in 7 days, 13 per cent in 10 days, and 17 per 

 cent in 14 days. 



The reduction of nitrates to nitrites has been used in 

 the United States as one of the five standard tests for 



