38 CONDITIONS OF BACTERIAL LIFE. 



Since the diagnostic value of the test of the resistance 

 to chemicals plaj's only a modest role — various hopes in 

 this direction remaining unfulfilled — this section must be 

 very brief. 



To determine the minimal concentration of a chemical 

 poison which produces asepsis — i. e. , prevents growth — the 

 following procedure is adopted: 



A solution of the disinfectant — for example, 1% — is em- 

 ployed; and of this, 1, 0.5, 0.3, 0.1 c.c. is added to lOc.c. 

 of liquefied gelatin. This nutrient medium, containing 

 now 0.1, 0.05, 0.03, 0.01% of the disinfectant, is used 

 for stab, streak, and plate cultures. Inoculations may 

 also be made with material containing only spores (ma- 

 terial freed from all bacilli by heating half an hour at 

 70°), and thus it may be determined whether the spores 

 grow out in cultures. 



Behring has devised the following practical method of 

 making these tests: A drop of fluid nutrient medium, — 

 for example, serum, — infected with the organism to be 

 tested, is removed before the addition of the antiseptic 

 and suspended from the under side of a cover-glass on a 

 hollow-ground slide, sealing it with a little vaselin (Tech- 

 nical Appendix). Then by degrees there is added to the 

 tube of serum, increasing known quantities of the dis- 

 infectant. After each addition and thorough shaking a 

 drop culture is prepared. The growth in each drop can 

 be examined after being kept twenty-four or forty-eight 

 hours in the incubator. 



If the concentration necessary for antisepsis is to be 

 determined, the organisms to be examined are grown in 

 bouillon, and to 10 c.c. of the Ijouillon, free from spores, 

 and filtered through asbestos to remove any clumps of 

 bacteria, various quantities of a solution of the disinfectant 

 of known strength are added. From each of the tubes 

 after one, five, ten, fifteen, thirty minutes, one hour, etc. 

 a platinum loopful of the mateiial is removed, and placed 

 in 10 c.c. of lukfwarm liquefied gelatin, from Avhich a 

 plate culture is prepared. If it is suspected that a trace 

 of the disinfectant carried in the drop renders the gelatin 

 aseptic and so leads to an ai)parent death of the micro- 

 organisms, the result may be controlled by inoculating 



