64 DISINFECTANTS 



come customary to standardize disinfectants not only 

 with the Gram-negative Bacterium typhosum, but also 

 with the Gram-positive Staphylococcus aureus. 



The degree of selectivity of various bacteria is not the 

 same with all disinfectants. Bacterium typhosum and 

 Staphylococcus aureus are killed by approximately equal 

 concentrations of acetic acid while, with certain soaps, a 

 concentration 30 times higher is required to kill the ty- 

 phoid bacteria than to kill the staphylococci. 



Table 13, which has been compiled from the data gath- 

 ered by McCulloch (1936), gives some conception of the 

 variations in the degree of selectivity. It shows the dif- 

 ferences in sensitivity of the two standard test species. 

 Small differences cannot be considered as due to selec- 

 tivity since we know that, even within the same species, 

 different strains may show remarkable variations of tol- 

 erance. Cases in which one of the test species required 

 at least 5 times as strong a disinfectant as the other have 

 been considered indicative of selectivity. Unfortunately, 

 the published data on selectivit}' show little agreement. 

 This is due, at least in part, to the inaccuracies in the 

 determination of the phenol coefficient (see p. 105). 



It is important for the theory of disinfection as well 

 as for the application of disinfectants that most of the 

 efficient, rapidly killing substances are not very selective. 

 However, hydrogen peroxide, permanganate, alkalis and 

 some soaps show a tendency towards selectivity. In the 

 case of alkalis, for instance, the typhoid bacterium is 

 killed in 10 minutes at pH 10.02 while the staphylococcus 

 requires a pH of 12.2, which represents 100 times as many 

 OH ions. 



Besides these differences between the standard test 

 species, differences occur also in a few other groups of 

 bacteria in regard to one or several disinfectants. Thus 

 the resistance of the tubercle group towards alkali and 

 chlorine is well-known. The waxy membrane of these 

 bacteria may explain their exceptional behavior. Not so 



