CHAPTER VII. 



ANTISEPTICS AND DISINFECTANTS. 



Many chemical substances have the power of entering into chem- 

 ical union with the protoplasm of bacterial cells and so forming new 

 compounds, and often coagulating the protoplasm. 



Bacteria differ in their powers to resist these agencies; the anthrax 

 spore is much more difficult to kill than the typhoid bacillus; these 

 chemical substances act at a high rather than a low temperature. 



A chemical disinfectant, such as copper sulphate, acts more 

 rapidly and effectively in a watery solution than in a complex 

 albuminous one. 



It is often necessary to determine the exact minimum amount of 

 an antiseptic that will destroy a given organism or produce a com- 

 plete inhibition of growth; for this purpose small amounts of a dis- 

 infectant are added to gelatine in test-tubes and these are poured 

 into plates and the result noted. 



Previous to pouring the plates each tube is inoculated with a 

 loopful of culture and thoroughly mixed with the medium. 



Another method is to make bouillon cultures of an organism and 

 add to each a certain percentage of the solution of the antiseptic, 

 and abstract every few minutes after the addition of the chemical 

 one loopful of the mixture and inoculate fresh media. 



It will be found in the case of most antiseptics in dilute solution 

 that an interval of time must elapse before the organisms are killed. 

 This is determined by observing the cultures made from the mixture. 

 After five minutes, growth may occur, but after one hour, all may be 

 dead, or it may take two or three hours. 



The most valuable chemical disinfectants are those that kill in 

 highly dilute solution in a short time. 



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