28 



Evidently scrupulous care must be employed in the preparation of 

 the medium to avoid introduction of any nitrogenous compounds which 

 might be present in imperfectly cleaned glass-ware or ordinary tap water 

 or even distilled water, for the nitrogen thus introduced might serve to pro- 

 mote growth of organisms of the coli subgroup and thus obscure the 

 differential reaction. 



The reliability of this uric acid differential test has been confirmed by 

 Chen and Rettger in a study of 640 strains isolated from soil and from 

 intestinal tracts of man and various animals. They note, however, that 

 half the coli strains isolated from the soil grew in the uric acid medium. 



TABLE XIII. GROWTH OF THE COLON GROUP IN URIC ACID MEDIUM 

 (After Chen and Retteer, 1920) 



They showed further that xanthine may be substituted for uric acid and 

 that all the coli strains from the soil failed to grow in the xanthine medium. 

 It is probable that xanthine will be found preferable to uric acid for dif- 

 ferentiation of the coli from the aerogenes types but there may be an 

 intermediate group resembling the former with respect to the V. P. and 

 methyl red reactions but which differs from it in its ability to attack uric 

 acid. 



The temperature of incubation employed by Koser was 37 degrees 

 C. for four days. Chen and Rettger employed a temperature of 30 degrees 

 C. for three to five days. 



Differentiation on Solid Media. The coli and aerogenes sections 

 also present several cultural differences particularly with respect to the 

 appearance of colonies on some solid media. The aerogenes colonies are 

 generally larger more opaque and more convex than those of the coli sub- 

 group. On litmus lactose agar the former sometimes revert to an alkaline 

 reaction. These differences, however, are very difficult to detect on litmus 

 lactose agar but may be readily observed in the Endo and particularly on 

 eosin-methylene-blue agar. 



Ferreira, Horta and Paredes noted that Bact. aerogenes and Bact. clo- 

 acae produced a rose color on Endo agar but that the metallic luster, so 

 characteristic of Bact. coli, was absent. Levine employing a simplified 

 fuchsin sulphite (Endo) agar notes striking differences between the col- 

 onies of the coli and aerogenes subgroups. The coli strains formed deep red 

 button-like colonies with a greenish metallic sheen (by reflected light), and 

 were usually three or four m. m. in diameter when well isolated. The 

 aerogenes colonies are lighter colored, markedly convex, and do not show 

 the metallic luster. 



