8o BACTERIOLOGY 



Testing Antiseptics and Disinfectants 



The determination of the antiseptic value of a material is a 

 comparatively simple mattef. A virulent culture of the organ- 

 ism used as a test is inoculated into sterile bouillon containing a 

 known quantity of the antiseptic. The process is repeated with 

 varying strengths of the material until the smallest 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 conditions of the test. 



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 substance tested remains 

 in contact with the organisms in the method given for determin- 

 ing the antiseptic value, and that we do not know whether the 

 bacteria are alive and merely inhibited in growth, or actually 

 killed. 



The chemical composition of the medium in which the bac- 

 teria are tested may have a marked influence upon the action of 

 germicides. If components of the medium enter into chemical 

 union with the germicide there may be an inert compound 

 formed. There may also be formed dense, flocculent precipi- 

 tates which envelop the bacteria and protect them from the action 

 of the germicide. It is therefore apparent that the potency of a 

 germicide may appear very different when acting upon the bac- 

 teria in water or in physiological salt solution or on bacteria 

 dried on glass rods or on silk threads, on the one hand, and upon 

 the same bacteria in beef broth or in feces or in urine, on the other. 

 For these reasons it is not always possible to draw conclusions 

 from the results of laboratory experiments as to the value of a 

 germicidal agent for practical disinfecting purposes. 



Method. — To 15 c.c. of sterile water in a 60 c.c. Erlenmeyer 

 flask add 2 c.c. of a virulent culture of the test-organism. Then 

 add a solution of the substance under investigation in the pro- 



