25 



attacking sucrose produce less acid than the V. P. positives. The means 

 for the two groups are .74 and 1.57 percent respectively and the empirical 

 modes 0.7 and 1.5 percent normal acid. 



Acid Production from Various Other Carbohydrates. Obser- 

 vations on the average quantities of acid formed from various substances 

 by members of the coli (V. P. ) and aerogenes (V. P. + ) subgroups, 

 obtained from the American Museum collection and those isolated by the 

 author from feces and sewage gave the results shown in Table X. 



Inspection of Table X. and figure 2 indicates that considering all of 

 the 167 strains studied, the aerogenes strains form less acid from glucose 

 than does the coli section and about equal quantities form galactose, manni- 

 tol, and lactose. In all other test substances, maltose, salicin, raffinose, 

 dulcitol, glycerol, and sucrose, the V. P. positive strains (aerogenes) give 

 rise to considerably more acid, the excess increasing in the order named. 

 Although the differences obtained in salicin, raffinose, and possibly glucose, 

 may not be so significant on account of the variation observed among in- 

 dividual strains, it is nevertheless quite striking that a small number of 

 strains taken at random, from the American Museum collection and a few 

 freshly isolated cultures should show such a marked parallelism as is in- 

 dicated in Table X. and figure 2 with respect to the amount of acid formed 

 when they decompose such a variety of carbohydrates. 



The H + ion Concentration. In 1915 Clark and Lubs first pointed 

 out that in a medium consisting of 0.5 percent anhydrous glucose, dipotas- 

 sium phosphate, and Witte's peptone, the low ratio cultures (coli section) 

 produced a high acidity (H + ion) which remained permament; the high 

 ratio strains (aerogenes section) were much less acid and became progress- 

 ively more alkaline. This difference in acidity was easily recognized by 

 the use of the indicator methyl red which gave an acid reaction with the 

 low ratio and an alkaline reaction with the high ratio group. This test 

 has become known as the methyl red reaction. 



Principle of the Methyl Red Reaction. Michaelis and Morcora* 

 observed that cultures of Bact. coli fermented lactose until a hydrogen ion 

 concentration of IXlO 5 was reached and then ceased their activity. They 

 considered this point a physiological constant for the organism in question. 

 Clark, in 1915, obtained similar results with a culture of Bact. coli in glu- 

 cose peptone water. He found that, irrespective of the initial acidity, the 

 final hydrogen ion concentration varied but slightly (PH 4.37 to 4.55). 

 If we again consider the products of decomposition of glucose by Bact. coli 

 and Bact. aerogenes (see Table VII.), it will be noted that whereas Bact. 

 aerogenes decomposes 14 percent of glucose into acids (acetic lactic, suc- 

 cinic, and formic) Bact. coli produces acids from 56 percent of the glu- 

 cose. Thus, from a given quantity of sugar, the amount of acid evolved 

 and consequently the H+ion reached will be greater for Bact. coli than for 

 Bact. aerogenes. If now the amount of glucose in a medium is restricted 

 to that quantity necessary to yield the limiting (inhibiting) H + ion con- 

 centration for Bact. coli, the resulting reaction with Bact. aerogenes will 



*Cited by Clark and Lubs 



