CHARACTERISTICS OF THE COLON-AEROGENES GROUP 85 



son 11 shows that the acids formed by the accepted types of 

 B. coli, and B. aerogenes are identical but that the amounts 

 of these acids produced by B. coll are less than those pro- 

 duced by B. aerogenes. The results obtained by Ayers and 

 Rupp show that the final difference is due primarily to dif- 

 ferences in the rates of the various fermentations. In the 

 low ratio type the acid formation proceeds faster than the 

 fermentation of the acid salts and a hydrogen ion concentra- 

 tion which inhibits growth may be reached. In the high ratio 

 group the fermentation of the acids is more rapid and the 

 initial acid reaction may be converted into an alkaline one. 

 With these facts before us it is easier to explain the more 

 complicated and more variable gas evolution of the high ratio 

 group. 



By carefully adjusting the amount of sugar and the buffer 

 action, Clark and Lubs 12 devised a medium in which the low 

 ratio cultures reached the limiting hydrogen ion concentra- 

 tion while the high ratio cultures fermented the greater part 

 of the sugar without reaching this point, thus permitting a 

 reversion of the reaction toward the alkaline side. The PH 

 value of 5.0 - 4.8 (Sorensen's scale), reached very uniformly 

 by the low ratio group, gives a red color with methyl red 

 while the high ratio cultures are uniformly yellow. Nearly 

 all of our cultures have been subjected to this test and it has 

 been found to agree with the gas determination in every case. 

 With only a few cultures was there any question as to the 

 reaction and in these cases it was found that there was some 

 abnormality in the gas ratio which would justify classing 

 these cultures as atypical. Other workers who have used 

 this test have also found it a reliable method of distinguishing 

 between the B. coli and B. aerogenes type, but some, who have 

 considered it less reliable than the Voges-Proskauer test, 

 have apparently failed to realize that it was designed to in- 



11 James Thompson, The Chemical Action of Bacillus Cloacae (Jordan) 

 on Glucose and Mannitol in Proc. Roy. Soc. (London, 1912), Ser. B, 84, 

 pp. 500-504. 



12 W. M, Clark and H. A. Lubs, Improved Chemical Methods for Differ- 

 entiating Bacteria of the Coli-Aerogenes Family in Jour. Biol. Chem., 

 30, 2, pp. 209-234, 1917. 



