COLON-AEROGENES GROUP OF BACTERIA 265 



The fallacy of the titrimetric method of determining the 

 amount of acid production by bacteria, and the necessity of the 

 application of more accurate and dependable methods for estimat- 

 ing the H ion concentration in culture media and in bacterial 

 cultures have been clearly demonstrated by Clark and by Clark 

 and Lubs (1915-1917). As a result of their important observa- 

 tions, the colorometric method which has been so advantageously 

 applied by others to biochemical problems in animal physiology 

 has quite generally become a routine part of bacteriological 

 procedure. 



B. Hydrogen ion concentration and the methyl red test 



The successful attempts of recent years to establish more 

 definite relationships of the important members of the colon 

 group with each other and to place their classification on a more 

 scientific basis were stimulated by the works of Harden (1901 

 and 1905), Harden and Walpole (1906), and Thompson (1911). 

 They found that the end products of glucose fermentation, such 

 as lactic, succinic, acetic and formic acids, and ethyl alcohol, 

 by B. coli differ in quantity from those which are formed by the 

 organisms now known as B. aerogenes and B. cloacae. Michaelis 

 and Marcora (1912) gave further evidence to show that there 

 is a "physiological constant" in B. coli cultures when this type 

 of organism is grown in lactose broth. This constant is evidenced 

 by the cessation of its activity at a definite hydrogen ion concen- 

 tration of 1 X 10- 5 . 



The Clark and Lubs (1915) phosphate-glucose-peptone medium 

 serves admirably to apply the above principles to practical use. 

 This, as well as the subsequent synthetic phthalate medium of 

 these investigators, furnishes the necessary conditions to permit 

 an indicator of the right choice to register the hydrogen ion 

 concentration within sharply defined limits, so that the distin- 

 guishing character of the indicator is not. altered or obscured. 

 Their selection of methyl red as the indicator of merit has also 

 done much to facilitate our study of these organisms. 



