CHAPTER XXVIII. 



Methods of testing disinfectants and antiseptics Experiments illustrating 

 the precautions to be taken Experiments in skin disinfection. 



THERE are several ways of determining the germicidal 

 value of chemical substances, the most common being 

 to expose organisms dried upon bits of silk thread to 

 the disinfectant for different lengths of time, and then, 

 after removing, and carefully washing the threads in 

 water, to place them in nutrient media at a favorable 

 temperature, and notice if any growth appears. If no 

 growth results the disinfection is presumably successful. 

 Another method is to mix fluid cultures of bacteria 

 with the disinfectant in varying proportions, and, after 

 different intervals of time, to determine if disinfection 

 is in progress by transferring a portion of the mixture 

 to nutrient media, just as in the other method of work. 



By the former process the bits of thread, usually 

 about 1 to 2 cm. long, are placed in a dry test-tube 

 provided with a cotton plug and carefully sterilized, 

 either by the dry method or in the steam sterilizer, 

 before using. They are then immersed in a pure 

 bouillon culture or in a salt solution suspension of the 

 organism upon which the disinfectant is to be tested. 

 I say pure culture because it is always desirable in 

 testing a new germicide to determine its value as such 

 on several different resistant species of bacteria, both in 

 the vegetating and in the spore stage. After the threads 

 have remained in the culture or suspension for from five 



