370 PHYSIOLOGY OF BACTERIA 



the chemical effect of the surface tension depressant 

 upon the cell to be the real cause of death. However 

 the measurements refer to the tension between medium 

 and air, and this need not be proportional to that between 

 cell and liquid. Pizarro (1927) also believes that 

 chemical reaction is the real cause of death. 



Some light has been thrown on this question by the 

 experiments of Frobisher (1927) who studied the effi- 

 ciency of disinfectants in the presence of chemically 

 inert surface tension depressants (soaps). He found 

 that 0.1% of sodium oleate had no effect on phenol 

 disinfection, but 0.25% increased the poisonous effect, 

 and 0.5% prevented it. The latter is explained by the 

 law of Gibbs and Thompson that surface tension depress- 

 ants are strongly adsorbed on the surfaces. The layer 

 of adsorbed soap around the cell may become so thick 

 that phenol cannot readily penetrate to the cell. An 

 excess of sodium oleate also decreased the action of 

 hexyl resorcinol. Hansen (1922) found also that saponin 

 or soap had frequently but little or no effect, and some- 

 times even counteracted the efficiency of disinfectants. 



Leonard and Feirer (1927) experimented with hexyl resorcinol 

 which was the most powerful depressant they could find, reducing 

 the surface tension of water from 73 to 34 dynes, but reducing that of 

 glycerol only from 72.5 to 67. In pure glycerol, this substance did 

 not kill bacteria readily while in water, a 0.1% solution killed bac- 

 teria in fifteen seconds. The authors claim the slower action in 

 glycerol to be a proof that surface tension depression is really the 

 killing agent. With more knowledge concerning the protective 

 action of concentrated solutions against other causes of death (see 

 p. 330), the lower death rate of bacteria in glycerol + hexyl resor- 

 cinol might be accounted for on a different basis. But the pro- 

 portionality between surface tension and disinfecting power of the 

 different substituted resorcinols is a good indication that here we are 

 dealing with a physical rather than a chemical cause of death. 



