162 



ANTISEPTICS 



Fig. 31. Multiplication curves of ^acvharomj/crs ellipsoideus treated 

 with various concentrations of benzoic acid, at pH 4.1. 



antiseptic. This is shown graphically in Fig 32 which 

 represents the effect of SO2 on Bacterium coli. The con- 

 centration of the added NaoSOs is plotted logarithmically 

 against the pH. The slanting dotted line is the border 

 line of tolerance. Above this line, the lag period is 

 infinite, and so is the generation time. Just below this 

 line, bacteria require 30 to 140 hours before growth be- 

 comes visible, and 2 to 4 times as many hours for doub- 

 ling their number. The generation times in this figure are 

 only relative values because multiplication was measured 

 by turbidity. 



A similar set of data for yeast with benzoic acid (Fig- 

 ure 33) where multiplication had been determined by 

 direct counts and plate counts, represents true generation 

 times. Here also, the lag period as well as the multipli- 

 cation rate are greatly affected. The line representing 

 the limit of tolerance has a slope different from that in 

 Figure 32. 



