104 ELEMENTS OF WATER BACTERIOLOGY 



(191 2) wisely concludes, " The entire group is typical 

 of the presence of faecal matter when water or sewage 

 examinations are to be considered." The Committee 

 defines the group as a whole by the following character- 

 istics: "Fermentation of dextrose and lactose with 

 gas production, short bacillus with rounded ends, 

 non-spore-forming, facultative anaerobe, gives positive 

 test with esculin, grows at 20° on gelatin and at 37° 

 on agar, non-liquefying in 14 days on gelatin. Gram- 

 staining negative." This definition again, however, 

 includes debatable elements. It seems to us very 

 doubtful whether there is sufficient evidence to warrant 

 making the esculin reaction a general criterion of the 

 colon group; and bacteria which liquefy gelatin more 

 or less slowly grade into otherwise identical non- 

 liquefying forms by almost imperceptible degrees. 



We hesitate to add another to the long list of arbitrary 

 definitions of the much-defined colon bacillus; but it 

 does seem to us important to get down to rock bottom. 

 From this standpoint we believe that the colon group 

 may be defined as including all aerobic non-spore- 

 forming bacilli which produce acid and gas in dextrose 

 and lactose media. For practical purposes the test 

 may be further reduced to positive reactions in a lac- 

 tose fermentation medium, growth on an aerobic agar 

 streak, and microscopic examination, since almost all 

 forms which ferment lactose ferment dextrose as well. 

 The relative importance of the various subdivisions of 

 this general group will be discussed in Chapter VIII. 



Isolation of Colon Bacilli by Direct Plating. The Wurtz 

 litmus-lactose-agar plate (Wurt^. 1802), as noted in 



