86 BIOLOGY AND TECHNIQUE 



are made to determine the presence of living bacteria and to distinguish 

 between inhibition or antisepsis and bacterial death or disinfection. 



The determination of the bactericidal or disinfectant value of a 

 chemical substance upon spores may be carried out by a variety of 

 methods. Koch,* using anthrax spores as the indicator, dried the spores 

 upon previously sterilized threads of silk. These were exposed to the 

 disinfectant at a definite temperature for varying times, the disinfect- 

 ant was then removed by washing in sterile water, and the threads 

 planted upon gelatin or blood serum media and incubated. A serious 

 objection to this method was pointed out by Geppert,^ who maintains 

 that it is impossible by simple washing to remove completely the disin- 

 fectant in which the thread has been soaked. This author suggests that, 

 whenever possible, the disinfectant, at the end of the time of exposure, 

 should be removed by chemical means. In the case of bichloride of mer- 

 cury Geppert exposes emulsions of the bacteria to aqueous solutions of 

 the disinfectant, and at the end of exposure precipitates out the bichlor- 

 ide of mercury with ammonium sulphide. In the case of a large number 

 of disinfectants, however, this is not possible, and, when the thread 

 method is used, removal of the chemical agent by washing must be 

 practised. Complete removal of the disinfectant is especially desirable, 

 since spores previously exposed to these substances are more easily in- 

 hibited by dilute solutions than are normal spores. The spores may be 

 dried upon the end of a glass rod, which, after exposure, is washed in 

 distilled water or salt solution and then immersed in sterile broth. ^ 



A simple method is that in which graded percentages of the disin- 

 fectant are added to the menstruum, blood, blood serum, broth, etc., in 

 which the disinfectant is to be tested, and equal quantities of bacteria 

 thoroughly emulsified in water or salt solution are added. Loopfuls of 

 these mixtures are then planted from time to time in agar or gelatin 

 plates upon which colony counts can afterward be made. 



In all such tests it is important to remember that the presence of 

 organic fluids, blood serum, mucus, etc., considerably alters the efficiency 

 of germicides, and whenever practical deductions are made, experimental 

 imitation of the actual conditions should be attempted. 



Practical Disinfection. — In practical disinfection with chemical 

 agents, the disinfectant must be chosen to a certain extent in accordance 

 with the material to be disinfected. 



1 Koch, Arb. a. d. kais. Gesundheitsamt, 1, 1881. 



2 Geppert, Berl. klin. Woch., xxvi, 1889. 



2 Hill, Rep. Am. Pub. Health Assn., xxiv, 1898. 



