CHAPTER II. 

 INHIBITION OF GROWTH. 



POISONS, GERMICIDES, DISINFECTANTS, ANTISEPTICS, PRESERVA- 

 TIVES. A great number of inorganic and organic bodies will destroy life 

 in comparatively weak solutions. These substances are called poisons 

 if they are considered in their effect upon man and animals. In their 

 application to microorganisms they are generally called germicides (germ- 

 killers), or disinfectants if the emphasis is laid upon the prevention of 

 infection rather than upon the actual killing of the microorganisms. 

 Analogous to the general term germicides, the terms bactericide and 

 fungicide are used occasionally. The term antiseptic means a prevention 

 of sepsis which may be accomplished by checking the growth without 

 necessarily killing all microorganisms. The meaning of the word pre- 

 servative is practically the same, only the latter is used more commonly 

 in relation to foods, feeding stuffs and preparations of similar origin while 

 the word antiseptic is largely used in relation to microbial diseases. A 

 strict line cannot be drawn between any of these definitions. A disin- 

 fectant, if diluted, becomes an antiseptic. A strong salt solution is an 

 antiseptic for some organisms and a disinfectant for others. Of the above 

 expressions, germicide is the most definite, but is not so commonly used 

 as the others. 



MODE OF ACTION. The action of a poison upon the cell is generally 

 considered an action upon the protoplasm. The poison is supposed to 

 combine chemically with the cell plasma producing compounds which 

 interfere with the continuation of the life processes and thus cause death. 

 If the cell has been subjected to the action of the poison only a short time, 

 it can be saved by removing the poison. Bacteria can be treated with 

 mercuric chloride (HgCl 2 ) so that they will no longer develop if trans- 

 ferred to a fresh medium. If the mercuric chloride is removed from the 

 cell by means of hydrogen sulphide, some of the organisms may be revived. 



The action of a disinfectant upon a suspension of bacteria is not uni- 

 form. The largest percentage of cells will be killed in a short time, 



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