STEEPTOCOCCUS HEMOLYTICUS 195 



must be carefully controlled. One example may serve to illus- 

 trate this point : H. M. Jones (1920) working with the various types 

 of pneumococci found that in a medium of Ph 7.0 no strain was 

 able to develop greater acidity than Ph 5.6, whereas if the initial 

 reaction was Ph 7.6 all strains gave a final hydrogen-ion concen- 

 tration ranging from 5.0 to 5.4. If the final Ph produced by 

 certain organisms is to serve a useful purpose in differential pro- 

 cedures, the level of the initial hydrogen-ion concentration of 

 the medium must obviously be controlled so as to permit the 

 optimum development of the organism in question, in order that 

 it may carry its fermentation to a maximum. 



That there are levels of hydrogen-ion concentration which have 

 the effect of limiting the activities of certain organisms was 

 perhaps first recognized by Lazarus (1908) in 1908, who roughly 

 adjusted her media to various hydrogen-ion concentrations with 

 litmus, phenolphthalein, and methyl orange after which the reac- 

 tions limiting growth were studied. The influence of reaction 

 was considered a modification of the conditions of assimilation 

 in that it exerted a definite effect upon the state of dissociation 

 of the materials which the organism in question could take up 

 or could alter. 



With the recognition by investigators of the growing impor- 

 tance of the relationships of hydrogen-ion concentration to bio- 

 logical process in general, have come attempts to determine the 

 limits of reaction within which bacteria may develop. The 

 most complete single effort to establish such minimum, maxi- 

 mum, and optimum limits of Pn for a number of pathogenic 

 organisms seems to have been that of Fennel and Fisher (1919). 

 In the course of the present investigation it has been possible to 

 collect from a number of sources data bearing on this point and 

 in recognition of the value of a compilation such as this to work- 

 ers in the field of bacteriology this information has been appended 

 to the present section of the paper. 



