CLASSIFICATION OF THE COLON-CLOACAE GROUP 261 



correlated with glycerol, dextrin, starch and gelatin. Each of 

 these characters is correlated with each other. Under these cir- 

 cumstances any of these reactions may be selected for subdivision; 

 the choice depending upon which were employed in an investiga- 

 tion and to some extent on the personal preference of the investi- 

 gator. The characterization of B. aerogenes by Durham as a 

 starch fermenter; the differentiation of B. aerogenes from B. 

 cloacae by MacConkey on gelatin liquefaction and motility, 

 and by Kligler on glycerol fermentation are all correct; the 

 apparent confusion being the inevitable result of separation upon 

 single characters. 



Two species are evidently present, the B. aerogenes which 

 rarely, if ever, liquefies gelatin; is non-motile; and forms gas 

 from glycerol and starch; and the B. cloacae which liquefies 

 gelatin (often very slowly) ; is motile; and does not form gas from 

 glycerol nor starch. As gelatin liquefaction is an inconvenient 

 character the organisms are subdivided for further study upon 

 motility into the non-motile B. aerogenes and the motile B. 

 cloacae. Glycerol or starch would do just as well. Whichever 

 character is selected, a few strains are present in each of the result- 

 ing groups which possess some of the salient characteristics of the 

 other. Thus of 89 motile strains 8 did not liquefy gelatin, 8 

 formed gas from glycerol and 4 from starch, while of 62 non- 

 motile strains, 2 liquefied gelatin, and glycerol and starch were 

 attacked by one. 



The presence of a few supposedly non-liquefiers among the 

 motile strains may as probably — and even more probably — be 

 an indication of the inaccuracy and unreliability of the gelatin 

 liquefaction test than of the presence of true intermediate organ- 

 isms, for the number of gelatin liquefiers recognized increases 

 with the period of incubation. Again is it not reasonable to 

 explain the presence of several glycerol and starch fermenters 

 among the motile strains as due to mixed cultures? Picking off a 

 colony from a plate, even after several replatings, is no absolute 

 criterion that a pure culture was obtained. Some species stick 

 tenaciously together. 



One of the motile starch fermenting strains referred to above 



