DISINFECTANTS 485 



necessary by plating methods, further subcultivations 

 upon carbohydrate media and agglutination experi- 

 ments. The sign ( + ) is used to indicate that growth 

 of the test organism occurred in the subcultivations, 

 and the sign ( ) to indicate that the test germs have 

 been destroyed and no subsequent growth has taken 

 place. 



METHOD. 



Apparatus Required: 



Sterile test-tubes (narrow, not exceeding 1.3 cm. diameter). 



Test-tube rack (Fig. 219). 



Sterile graduated pipettes in case, i c.c. (in tenths). 



Sterile graduated pipettes in case, 5 c. c.(m c.c.). 



Circular rubber washers, 2.5 cm. diameter with central hole, 

 sterilised by boiling immediately before use, then transferred to 

 sterilised glass double dish. 



Electric signal clock or stop watch. 



Sterile forceps. 



Sterilised glass beads. . 



Shaking machine. 



Grease pencil. 

 Material Required: 



Percentage solutions of germicide-x (vide page 481). 



Percentage solutions of pure phenol (vide page 482). 



Aqueous emulsion of B. coli (vide page 481). 



Tubes of bile salt broth. 



Preliminary Tests. 



a. Inhibition Coefficient. 



Determine the lowest percentage of germicide-x 

 which inhibits growth of B. coli in the bile salt broth, 

 and the highest percentage which fails to inhibit (page 

 311). On the result of this experiment determine the 

 bulk of medium required in the subculture tubes and 

 the percentage solutions to be employed in the trial 

 trip. Assuming the inhibition coefficient to be i : 1000, 

 it will be quite safe to employ the ordinary culture 

 tubes containing 10 c.c. medium in the subsequent 

 experiments. 



