QQ DISINFECTANTS 



simply explained is the high degree of tolerance toward 

 iodine exhibited by Pseudomonas pyocyanea which, on 

 the other hand, is unusually sensitive to acids. Another 

 remarkable instance of selectivity is that of colloidal sul- 

 fur which affects the two test species almost equally, but 

 kills certain plant pathogens at one-tenth or one-thirtieth 

 of the usual concentration. 



With weak disinfectants, such as soap, and with many 

 antiseptics, the difference in tolerance of the two stand- 

 ard test species may be hundredfold. Among the out- 

 standing examples of extreme selectivity are some weak 

 acids like sulfur dioxide, the dyes, the sulfa drugs, and 

 many chemotherapeutic agents. 



From these observations, certain general conclusions 

 concerning the mechanism of death can be drawn. If a 

 disinfectant shows no selectivity, it causes death either 

 by reacting with a cell constituent common to all spe- 

 cies, or by causing a reaction which is common to all 

 proteins and which proceeds at the same rate in all 

 species. If a substance shows pronounced selectivity, it 

 reacts on cell constituents which are not alike in the 

 bacterial species concerned. 



II. CONCENTRATION AND DEATH RATE 



A classical example of the effect of concentration of 

 the disinfectant on death is shown in Table 14, which rep- 

 resents the action of HgCL on the spores of the anthrax 

 bacillus, and is taken from the first quantitative study 

 of chemical disinfection by Kronig and Paul in 1897. 



The data of this table may be anah^zed on the assump- 

 tion that the course of death of bacteria is described by 

 the equation 



initial number 



Kt=log 



survivors 



