194 MANUAL OF BACTERIOLOGY. 



quantity of it capable of preventing growth is determined. 

 This dilution may be considered the antiseptic value of the 

 material in question for the organism used, under the condi- 

 tions of the test. 



Determination of the germicidal pov?er of a substance is a prob- 

 lem of greater difficulty, and the method used must be adapted 

 to the requirements in each case. It is obvious that some 

 of the substance tested remains in contact with the organ- 

 isms in the method given for determining the antiseptic value, 

 and that we do not know whether the bacteria are ahve and 

 merely inhibited in growth, or actually killed in such tests. 



Sternberg's Method. — To a measured quantity of a virulent 

 bouillon-culture of the test-organism is added a given amount 

 of the germicide. After varying lengths of time inoculations 

 are made from this mixture into culture-media, preferably 

 bouillon, and note is made of the presence or absence of 

 growth under suitable conditions of temperature and the like. 

 The shortest exposure to the weakest solution of the substance 

 necessary to kill the test-organism is taken as the germicidal 

 value of that substance for the particular organism used. 



Koch's Method. — This method is 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-culture of the test- 

 organisms. The threads are then removed, partially dried and 

 placed in a solution of known strength of the germicide, and 

 exposed for a definite length of time. The thread is removed 

 from the solution, washed carefully in sterile water, dropped 

 into a tube of sterile bouillon plugged with cotton, and growth 

 or absence of growth noted. 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. 



