DISINFECTANTS AND ANTISEPTICS. IQ5 



Determination of the disinfectant power of a substance 

 is a problem of much greater magnitude, and the method 

 used must be altered more or less to suit the properties of 

 the substance tested. It is obvious that some of the sub- 

 stance tested remains in contact with the organisms in the 

 method given for determining the antiseptic value, and 

 that we do not know whether the bacteria are alive and 

 merely inhibited in growth, or actually killed. 



Sternbergs Method. To a measured quantity of a 

 virulent bouillon-culture of the test-organism is added a 

 known quantity of the substance to be tested. After vary- 

 ing lengths of time inoculations are made from this mixture 

 into culture-media, preferably bouillon, and growth watched 

 for under suitable conditions as to temperature and the like. 

 The shortest exposure of the test-organism to the smallest 

 quantity of the substance is taken as the germicidal value 

 of that substance for the particular organism used. 



Koch's Method. Usually employed for spore-bearing 

 bacteria like the bacillus of anthrax. The hay bacillus is 

 convenient to use when experiments are being made by 

 large classes of students. Small pieces of sterile silk or 

 cotton thread are soaked for some hours in a bouillon-cul- 

 ture of the test-organisms. They are removed, partially 

 dried, and then placed in a solution of known strength of 

 the substance being tested, and exposed for a definite 

 length of time. The thread is removed from the solution, 

 washed carefully in sterile water, planted in bouillon, and 

 growth is watched for. As in other methods, the greatest 

 dilution of the germicide that will kill the test-organism in 

 the shortest time is taken as the germicidal value of that 

 substance for the organism used. 



Hill's Method. The test organism is dried upon the end 

 of a sterile glass rod contained in a sterile test-tube, the end 

 of the rod projecting through a cotton plug. The end of 

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