DISINFECTION STUDIES 213 



of getting a closer view, perhaps, of one of the mechanisms in- 

 volved in bacterial death. In this connection might be mentioned 

 the contribution of Watson (1908) who showed that variation 

 in the rate of disinfection due to change in concentration of the 

 disinfectant could be expressed as an exponential function of the 

 concentration. 



Another apparent effect of increasing temperature is to de- 

 crease the period of induction j)rior to the logarithmic rate of 

 death. The evidence is incomplete, but it would seem that the 

 duration of induction varies in inverse proportion to some 

 exponent of the temperature. 



The effect of pH upon bacterial viability 



We have seen that an uncontrolled hydrogen ion concentra- 

 tion may affect the death rate variably in unbuffered surround- 

 ings, so much so as to obscure the effects of wide ranges of tem- 

 perature. When the pH is controlled by means of dilute buffer 

 solutions the death rates become stabilized. Under the latter 

 conditions there becomes evident a zone of pH in which the 

 death rates of the bacteria are at a minimum. For Bad. ty- 

 phosum this zone Ues between pH 5.0 and pH 6.4 and for Bad. 

 coli it is wider, with the optimum near absolute neutraUty (pH 

 7.0). These zones may be regarded as optima for tolerance 

 xinder moderate lethal conditions and when compared with the 

 optima for growth show interesting relations, especially in the 

 case of Bact. typhosuvi. The pH zone for optimum growth of 

 this organism Ues between 6.2 and 7.2, while here we have found 

 that the zone for greatest tolerance lies between pH 5.0 and 6.4. 

 Cohen and Clark (1919) showed for the colon-dysentery group 

 that a slight increase in acidity beyond the optimum limit for 

 growth caused a very large effect by preventing growth. The 

 same phenomenon has been found in the present study of response 

 to lethal conditions, where a slight increase in acidity caused a 

 prompt change from maxunum tolerance to high mortality. 



In practical disinfection the intensity of attack, presumably 

 is so great that such variations in tolerance as we have observed 

 would hardly affect the bacterial death rates. Yet the possibility 



JOURNAL OW BACTERIOLOOT, VOL. VII, NO. 2 



