DISINFECTANTS 



113 



The power of a disinfectant to kill bacteria is dependent in a 

 remarkable degree upon the nature of the medium in which bacteria 

 are present when the germicide is applied. Almost invariably the 

 greatest germicidal activity is shown when the substance acts upon 

 the bacteria freed from all contaminating culture media and sus- 

 pended in distilled water or salt solution. The presence of proteins, 

 peptones, and similar substances usually cause a great reduction in 

 the germicidal powers of the substance. This is also the case in the 

 presence of pus, many of the organisms being partly digested in 

 the body of dead leukocytes. This property is illustrated by the 

 following table reported by Dakin and Dunham. The - - sign 

 indicates sterilization as indicated by negative subcultures, and the 

 + sign incomplete sterilization. 



This decreased efficiency in the presence of a protein is variously 

 explained. In the case of such disinfectants as phenol and the dye- 

 stuffs, it is frequently stated that the disinfectant is "quenched" 

 or "fixed" by the protein medium, Adsorption in some cases may 

 play a part, but in the case of salts of the heavy metals, they com- 

 bine with the protein giving an insoluble non-ionizing proteinate. 

 The low germicidal action shown by most antiseptics against pus 

 is due in part no doubt to the mechanical difficulties of penetrating 

 the mucoid particles in the pus. 



Young cultures of bacteria are usually more resistant than are 

 older cultures. This is especially true when the disinfectant is 

 applied to cultures living in the products resulting from their 

 metabolism. Cultures, the organisms of which form spores, become 

 more resistant to disinfectants as the spore stage is reached. 



Emulsions as a rule have greater germicidal power than have 

 solutions. According to Chick and Martin, emulsions or soapy 



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