DISINFECTION, STERILIZATION AND ANTISEPSIS. 241 



terium under observation. It is stronger in the cold. The ex- 

 planation offered for this behavior is that the vigor of growth 

 at the optimum temperature overcomes the inhibitory action 

 of the germicide, whereas at lower temperatures the vigor of 

 the organism is lowered, and therefore it is less capable of 

 resisting the inhibitory action than at the more suitable tem- 

 perature for growth. 



The chemical composition of the medium in which the 

 bacteria are tested may have a marked influence upon the 

 action of germicides. If components of the medium enter into 

 chemical union with the germicide there may be an inert 

 compound formed. There may also be formed dense, floccu- 

 lent precipitates which envelop the bacteria and protect them 

 from the action of the germicide. It is therefore apparent that 

 the potency of a germicide may appear very different when 

 acting upon the bacteria in water or in physiological solution 

 or on bacteria dried on glass rods or on silk threads, on the 

 one hand, and upon the same bacteria in beef -broth or in feces 

 or in urine, on the other. For these reasons it is not always 

 possible to draw conclusions from the results of laboratory 

 experiments as to the value of a germicidal agent for practical 

 disinfecting purposes. 



The action of germicides is still further complicated by the 

 difference of resistance shown not only by different species of 

 bacteria, but by the different strains of the same bacterium and 

 even by different individuals in the same culture. Further- 

 more, some bacteria are more sensitive than others to certain 

 chemical agents. In other words, some bacteria have an elec- 

 tive af&nity for certain chemicals while others have greater 

 affinity for other chemical agents. As examples of this elec- 

 tive affinity of certain bacteria for certain germicides, Got- 

 schlich* cites the action of quinine in malaria, the mercury 

 salts in syphilis, and the specific action of bactericidal sera. 



*Loc. cit. 



