THE DESTRUCTION OF BACTERIA 105 



The determination of the bactericidal or disinfectant value of 

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

 of methods. Koch, 60 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 disinfectant 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, 61 who maintains that it is impossible by simple washing 

 to remove completely the disinfectant in which the thread has been 

 soaked. This author suggests that, whenever possible, the disin- 

 fectant, at the end of the time of exposure, should be removed by 

 chemical means. In the case of bichloride of mercury, Geppert ex- 

 poses emulsions of the bacteria to aqueous solutions of the disin- 

 fectant, and at the end of exposure precipitates out the bichloride 

 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 

 practiced. Complete removal of the disinfectant is especially de- 

 sirable, since spores previously exposed to these substances are more 

 easily inhibited 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. 62 



A simple method is that in which graded percentages of the 

 disinfectant 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. 



Gaseous Disinfectants for Purposes of Fumigation. There are a 

 large number of gaseous agents which are harmful to bacteria. Only 



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



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



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



