CHAPTER XVll 



DETERMINATION OF THE VALUE OF ANTISEPTICS, 

 GERMICIDES, AND DISINFECTANTS 



The student must bear in mind that an antiseptic is a substance 

 capable of restraining the growth of bacteria; a germicide, one ca- 

 pable of killing them. All germicides are antiseptic in dilute solu- 

 tions, but not all antiseptics are germicides. Disinfectants must be 

 germicides. 



Antiseptics are chiefly employed for purposes of preservation, 

 and are largely used in the industries to protect organic substances 

 from the micro-organisms of fermentation and decomposition. The 

 problem is to secure a satisfactory effect with the addition of the 

 least possible preservative in order that its presence shall not chem- 

 ically destroy the good qualities of the substances preserved. In 

 the case of foods it becomes necessary to use preservatives free froni 

 poisonous properties. 



Disinfectants and germicides are employed for the purpose of 

 destroying germs of all kinds, and the chief problem is to secure 

 efficiency of action, rather than to endeavor to save on the reagent, 

 which would be a false economy, in that the very object desired might 

 be defeated. 



The following methods of determining the antiseptic and germi- 

 cidal values of various agents can be elaborated according to the 

 extent and thoroughness of the investigation to be made. 



I. The Antiseptic Value.^ — Remembering that an antiseptic is a 

 substance that inhibits bacterial growth, the determination of its 

 value can be made by adding varying quantities of the antiseptic 

 to be investigated to culture-media in which bacteria are subse- 

 quently planted. It is always well to use a considerable number of 

 tubes of bouUlon containing varying strengths of the reagent to be 

 investigated. If the antiseptic be non-volatile, it may be added 

 ■ before sterilization, which is to be preferred; but if volatile, it must 

 be added by means of a sterile pipet, with the greatest precaution 

 as regards asepsis, after sterilization and immediately before the test 

 is made. Control experiments — i.e., bouillon cultures without the 

 addition of the antiseptic — should always be made. 



The residts of antiseptic action are two: retardation of growth and 

 complete inhibition of growth. As the inoculated tubes containing 

 the antiseptic are watched in their development, it will usually be 

 observed that those containing very small quantities develop al- 

 most as rapidly as the control tubes; those containing more, a little 



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