DISINFECTANTS 481 



bacillus is an organism which is slightly more resistant 

 to the lethal action of germicides than the more patho- 

 genic members of this group, a margin of safety is in- 

 troduced into the test which certainly enhances its value. 

 B. coli should be recently isolated from a normal 

 stool, and plated at least twice to ensure the purity of 

 the strain; and a stock agar culture prepared which 

 should be used throughout any particular test. For any 

 particular experiment prepare a smear culture on agar 

 and incubate at 3 7 C. for 2 4 hours anaerobically. Then 

 emulsify the whole of the surface growth in 10 c.c. of 

 sterile water. Transfer the emulsion to a sterile test- 

 tube with some sterile glass beads and shake thoroughly 

 to ensure homogenous emulsion. Transfer to a centri- 

 fuge tube and centrifugalise the emulsion to throw 

 down any masses of bacteria which may have escaped 

 the disintegrating action of the beads. Pipette off the 

 supernatant emulsion for use in the test. 



2. Germicide. 



a. Disinfectant to be tested. 



The first essential point is to test the unknown disin- 

 fectant, which may be referred to as germicide-x, on 

 the lines set out on page 3 1 1 to determine its inhibition 

 coefficient. 



This constant having been fixed, prepare various solu- 

 tions of germicide-x with sterilised distilled water by ac- 

 curate volumetric methods, commencing with a solution 

 somewhat stronger than that representing the inhibition 

 coefficient. The solutions must be prepared in fairly 

 large bulk, not less than 5 c.c. of the disinfectant being 

 utilised for the preparation of any given percentage 

 solution. 



b. Standard Control. Phenol. 



The standard germicide used for comparison should 

 be one which is not subject to variation in its chemical 

 composition, and the one which has obtained almost 

 universal use is Phenol. r 



