50 LABORATORY EXERCISES IN BACTERIOLOGY. 



being then inoculated with the test bacteria. That tube containing the least amount of 

 the antiseptic in which growth fails to appear represents the antiseptic value of that 

 substance in relation to the particular bacterium used in the test. However, in such 

 tube it should be possible, upon the addition of an excess of the peptone solution (thus 

 further diluting the antiseptic), to obtain the characteristic development of the organ- 

 ism. 



Exercise 17. Four tubes, each containing four cubic centimeters of 

 a one per cent, solution of peptone, are prepared. To the first is added 

 one cubic centimeter of a two per cent, solution of borax (making a i : 250 

 solution of the borax in the tube] ; to the second, one cubic centimeter of a 

 one per cent, solution of borax (i : 500 solution in tube} ; to the third, one 

 cubic centimeter of a half per cent, solution of the borax (i : 1000 solu- 

 tion in tube) ; to the fourth, one cubic centimeter of a quarter per cent, 

 solution of the borax (i 12000 solution in the tube). Inoculate each tube 

 with a virulent culture of the colon bacillus, and incubate for three days. 

 As a control, a tube of the same medium is to be inoculated at the same 

 time with the germ, no borax having been introduced. Note the ap- 

 pearances presented by each tube at the close of each twenty-four hours. 

 At the end of the third day select the tube which fails to show any bac- 

 terial contamination, haying the least proportion of borax present. Di- 

 lute the contents with an equal amount of sterile peptone solution and 

 again incubate. Growth should now follow. Should it fail it is to be 

 presumed that the proportion of borax contained therein has acted rather 

 as a disinfectant; and the tube containing the next lower proportion of 

 the borax should be similarly treated to determine the persistence of 

 vitality of the bacilli. The first tube of the series which thus presents 

 growth after dilution with more of the culture medium, but which before 

 dilution failed to exhibit development of the organisms which had been 

 transplanted therein, represents the antiseptic value of borax for the 

 colon bacillus. 



Exercise 18. To demonstrate the possible failure of a disinfectant 

 because of the formation of a protective layer of organic compounds of 

 the disinfectant with the nutrient medium or with the outer portion of 

 the bacterial wall, thus encasing the bacteria or their spores and pre- 

 venting proper contact between the disinfectant and the microorganismal 

 body, the following exercise should be practised : Suspend for several 

 hours five bits of sterilized silk or cotton thread, each about two inches 

 in length, in a bouillon culture of the hay bacillus (B. subtilis), which 

 has been grown in the incubator for three or four days. The threads 

 after removal are suspended in an empty sterile tube and dried at incu- 

 bator temperature. They are next suspended in a i : 1000 solution of 



