194 LAURENCE F. FOSTER 



8. The initial Ph of broth exerts an effect upon the rate of 

 acid formation, A medium of Ph 7.5 was found to show a mini- 

 mum of lag, while the most rapid acid formation occurred in 

 broth of Ph 8.1. The optimum Ph of broth for growth and 

 acid production of the "H" strain of Streptococcus hemolyticus 

 apparently lies between these two points, Ph 7.5-8.1. Other 

 observers have fixed the optimum Ph of the streptococcus at 7.8. 



IV. THE INFLUENCE OF THE INITIAL Ph OF BROTH UPON GROWTH 

 AND ACID FORMATION OF STREPTOCOCCUS HEMOLYTICUS 



Before the elaboration of accurate methods for determining 

 the true reaction of a medium much attention was given to the 

 study of the influence of acidity and alkalinity upon the physio- 

 logical activities of organisms. Unfortunately much of the 

 data obtained in these earlier investigations is of little value 

 owing to the fact that determinations of titratable acidity rather 

 than of true acidity were carried out. The fallacy of titrating 

 media by the older method has been established by Clark (1915a) 

 beyond question and if we are to accept the classic works of 

 Sorensen and Michaelis, as supplemented by a constantly 

 increasing mass of data by other investigators, it must be sup- 

 posed that the hydrogen-ion concentration rather than the titrat- 

 able acidity of the environmental medium is the determining 

 factor in regulating the metabohc activities of bacteria and 

 related organisms. 



Though it is true that media adjusted by the old titration 

 method may vary considerably in their hydrogen-ion concen- 

 trations yet it has been possible in the past to cultivate bacteria 

 with a considerable degree of success. No doubt this has been 

 due rather to the fact that many bacteria are able to develop 

 within a fairly wide range of reaction than to the accuracy of 

 adjustment of the media. The effect of variations in initial Ph 

 would be demonstrable rather in altered rates of growth and 

 fermentation. In the case of some of the more dehcate patho- 

 genic bacteria, small variations in reaction may induce very 

 decided effects and it is here particularly that the true reaction 



