178 LAUEENCE F. FOSTER 



In both of these investigations it is evident that the maximum 

 changes in acid formation take place simultaneously with a 

 rapid development and multiplication of the bacteria and thus 

 show a conformity with the conception of Slator (1916) that 

 ''Chemical reactions brought about by microorganisms usually 

 proceed under conditions where development of the organism 

 and changes in the composition of the nutrient medium take 

 place simultaneously." H. M. Jones (1920a) however, has 

 recently obtained results which contradict the work of Cullen 

 and Chesney, and Lord and Nye. Using cultures of pneumo- 

 cocci in glucose broth this investigator has shown that the growth 

 curve rises sharply at the fourth to fifth hour while the onset of 

 the maximum period of acid formation is delayed until the 

 twelfth hour. Examination of the curves of this experiment 

 shows the maximum period of growth to be associated with but 

 a slight alteration in the reaction of the medium (7.4-7.0), 

 whereas, the interval of acid formation at a maximum rate corre- 

 sponds with the period of growth at a decreasing rate. This 

 finding corresponds more or less closely to the observations of 

 Cohen and Clark (1918) upon Bad. coli in glucose broth cultures. 

 The growth curve was found to rise five hours previous to the 

 onset of the maximum period of acid production, and, as in 

 the experiments of Jones, the maximum period of acid formation 

 was found to be coincident with the period of growth at a decreas- 

 ing rate. At the point where strong symptoms of growth inhibi- 

 tion appeared, the Ph was found to correspond to the region at 

 which acetic acid had been previously shown to check growth 

 (5.5-5.7). The fermentative activity, however, was not seri- 

 ously checked until the culture approached the region in which 

 HCl had been found to inhibit growth (4.6-5.0). From a con- 

 sideration of these findings it will appear, in the cases of Bad. 

 coli and the pneumococcus at least, that the hydrogen-ion con- 

 centration may exert independent effects upon growth, on the 

 one hand, and upon acid formation on the other, so that in 

 experiments designed to follow the acid production of organisms 

 in carbohydrate media it will be unsafe to assume that maximum 

 changes in reaction parallel maximum rates of multiplication of 

 bacterial cells. 



