180 MASS. EXPERIMENT STATION BULLETIN 167. 



After the hydrogen ion concentration has been adjusted the flasks are 

 placed in an incubator at 37° C. for twenty-four hours, to establish their 

 steriUty as well as to maintain the medium at this temperature. After this 

 period 1 cubic centimeter of a forty-eight-hour lactose broth culture of 

 each, Streptococcus erysipelatis and Streptococcus lacticus, was used to 

 inoculate Set 1 and Set 2, respectively. Observations of rate of growth, 

 hydrogen ion concentration and proteolysis were made at the various 

 intervals as indicated in tables. 



Rate of Growth. 



It seemed essential to determine the rate of growth in connection with 

 the other factors. This was done macroscopically, by taking a degree of 

 turbidity ^ as an indicator. The results are recorded in Table 2. 



Table 2. — Rate of Growth. 



— none. + slight. ++ moderate. 



Set 1 — Streptococcus erysipelatis. 

 Set 2 — Streptococcus lacticus. 



+++ abundant. 



This table indicates that Streptococcus erysipelatis grows best in the 

 medium of which the hydrogen ion concentration is 2.24X10"^ or opti- 

 mum; the growth becomes slower as the hydrogen ion concentration 

 decreases; an increase of the hydrogen ion concentration seems to have 

 some deleterious influence upon the growth, viz., (H) = 1.59X10"^ has 

 lethal influence. On the other hand, Streptococcus lacticus grows very 

 slowly in all the media except (H) = 2.35X10'^°. This is interesting to 

 note because it suggests stimulation by preceding environmental condi- 



' This has been described in the previous work. 



