STREPTOCOCCUS HEMOLYTICUS 179 



Clark (1915b), working with BacL coli, was perhaps the first 

 to follow reaction changes in bacterial cultures by means of 

 determinations of hydrogen-ion concentration. No change in 

 Ph was noted under a period of ten hours in his experiment. 

 Itano (1916a) followed the changes in acidity in cultures of 

 B. subtilis and noted in certain media of unfavorable initial Ph 

 that an '' automatic adjustment" toward a more favorable reaction 

 occurred during incubation. Working with Clostridium perfringens 

 {C. welchii) and C. sporogenes (Metchnikoff), Wolf and Harris 

 (1917b) found that curves of acidity change followed closely 

 those of amino acid formation and gas production. Avery and 

 Cullen (1919b) used media of different initial Ph with pneumo- 

 cocci and demonstrated that after completion of lag, growth, as 

 evidenced by the rate of reaction change, proceeded at about 

 equal speeds. Neither the final Ph nor the rate of acid for- 

 mation was affected by the use of various available mono- or 

 di-saccharides. The maximum period was found to lie between 

 the fourth and eighth hours following seeding. Bunker (1919) 

 noted an initial acidity rise followed by alkaline reversion in cul- 

 tures of Corynebact. diphtheriae and apparently has shown that 

 toxin production is closely associated with this phenomenon, as 

 no toxin could be demonstrated in cultures which failed to exhibit 

 an alkaline reversion. In a study of the logarithmic or maxi- 

 mum period in cultures of several organisms by Cohen and 

 Clark (1918) it was observed that bacteria may multiply rapidly 

 for a time in media varying considerably in initial reaction. 

 The maximum period of growth in the case of Bad. coli fell 

 between the fifth and tenth hours. Schoenholz and Meyer 

 (1919), in their work on Bad. typhosum, have reported changes 

 in the growth curve through the influence of hydrogen-ion con- 

 centration. Thus they found that growth at a maximum rate 

 set in five hours following incubation, if the initial Ph of the 

 medium was 7.0. At lower and higher levels lag was of longer 

 duration. 



Avery and Cullen (1919a), using streptococci of human and 

 bovine origin, found the greatest increase in acidity between the 

 seventh and twelfth hours, using eighteen-hour cultures as 



