14 GENERAL MICROBIOLOGY 



B. CHEMICAL AGENTS 



I. Sterilization by Disinfectants. Sterilization by dis- 

 infectants has but limited use in bacteriological work. 

 The amount of disinfectant necessary to destroy existing 

 organisms in a nutrient medium is greater than the amount 

 necessary to inhibit multiplication of an organism which 

 may subsequently be used as an inoculum; the medium 

 is therefore rendered useless. 



1. Disinfectants may be used for any apparatus which 

 will not come in direct contact with culture media or with 

 the organisms under investigation. Fixed non-volatile dis- 

 infectants must be employed, since the vapors given off by 

 volatile compounds hinder the growth of organisms on 

 culture media. 



2. Disinfectants are in general use for sterilizing the 

 hands, woodwork, for washing out vessels and sterilizing 

 instruments during inoculation and other experiments. 



As an example, 1-1000 mercuric chloride, 1.5% formalin, 

 5% phenol, 2% compound solution of cresol, etc., are cheap 

 and adaptable in many cases. Tincture of iodin is valuable 

 for painting wounds. 



The common soaps, and more particularly green soap, 

 have a plight germicidal value, and this in conjunction 

 with their solvent action upon fats and protein, and the 

 mechanical cleansing which accompanies their use, justifies 

 assigning them an important place among the chemical 

 disinfectants. 



Disinfectants used for sterilizing the skin before col- 

 lecting pus, blood, etc., from the living subject must be 

 carefully removed by washing the part well with alcohol 

 before collecting material, otherwise the presence of the 

 disinfectant would materially interfere with the subsequent 

 growth of organisms in the culture. 



3. Disinfectants are also added to sterile filtrates which 

 are no longer required as culture media. For this pur- 



