Il8 LABORATORY BACTERIOLOGY 



EXERCISE LIII 



DETERMINING THE EFFICIENCY OF DISINFECTANTS 



171. The efficiency of the more commonly used disinfect- 

 ants has been determined for most of the pathogenic bacteria, 

 but new disinfectants are constantly being put upon the mar- 

 ket, and before it is safe to use or recommend them, their effi- 

 ciency should be determined. With many of the disinfectants, 

 such as carbolic acid, corrosive sublimate, lime, and the min- 

 eral acids, much stronger solutions are commonly used than 

 are actually necessary to kill the bacteria, owing to the fact 

 that frequently it is necessary to allow for an indefinite waste 

 due to the union of the disinfectant with other substances, 

 usually organic, with which the bacteria are mixed. For the 

 different methods of testing the efficiency of disinfectants, see 

 text-books. A very simple process is given here. 



It may be desirable for students to work in groups of two or 

 more in order to economize in the number of tubes required. 

 If possible, however, each student should make all of the tests. 



REFERENCES. Young, Notes on Disinfectants and Disinfec- 

 tion, Augusta, 1898. Rideal, Disinfection and Disinfectants, 

 London. Rosenau, Disinfection and Disinfectants. See also 

 text-books. 



172. Work for this exercise. Put 10 cc. of a 2% solution 

 of carbolic acid, prepared from sterile distilled water, into each 

 of 2 sterile test tubes. Add to one of these tubes, by means of 

 a sterilized pipette, .25 cc. of a bouillon culture of B. cholera 

 suis or B. typhosus. To the other tube add a like quantity 

 of a suspension in bouillon or sterile water of an agar culture 

 of B. subtilis (furnished). 



Inoculate a tube of bouillon containing fully 7 cc. with 6 

 loopfuls from each of these tubes after the expiration of the 



