TESTING THE EFFECT OF TEMPERATURE 185 



bacteria are not considered in disinfection. The destruction of all 

 life in a medium, as has already been explained, is called sterilization; 

 disinfection, however, does not go to this extent. 



The determination of the exact disinfecting value of either physi- 

 cal or chemical agents is more difficult than would appear on first 

 sight. It is always necessary in such experiments to use bacteria 

 which have been grown under the most favorable conditions and 

 which are derived from a young, vigorous culture. Another important 

 factor is whether these bacteria are contained in distilled water, salt 

 solution, or a favorable fluid culture medium, because in the latter 

 they are generally more resistant than in pure water. It is also 

 necessary to protect them from any damaging influences before the 

 actual test. 



Theobald Smith has shown that tetanus spores obtained under the 

 most favorable conditions can resist moist heat for over one hour, 

 while those raised in media containing sugar and previously damaged 

 by the acid which they have formed from it are much less resistant. 



The action of the disinfectant is also much influenced by the 

 presence of certain substances in the culture medium. Corrosive 

 sublimate, for example, is much weakened in the presence of albumin- 

 oid or proteid bodies or discharges containing them. After the test 

 the removal of the disinfectant from the bacteria by washing or 

 chemical means is another important factor. 



Testing the Effect of Temperature. The method used for testing 

 the effect of temperature upon bacteria is the following: Culture 

 tubes containing a few cubic centimeters of young bouillon cultures 

 of the organism are removed from the incubator and placed in a 

 water bath containing a large amount of water heated to a certain 

 stationary temperature. One of the tubes must contain a thermometer 

 in order to indicate the exact moment when the temperature in the 

 culture medium coincides with that of the water bath. The tubes are 

 then heated for varying periods of time for example, for five, seven, 

 ten, eleven, and twelve minutes, and so on. They are taken out of 

 the water bath one by one and at once plunged into cold water. 

 Upon the conclusion of this process, culture tubes with liquefied solid 

 media are inoculated, plates are poured, and the developing colonies 

 are subsequently counted. The first plate remaining sterile indicates 

 the time exposure necessary to kill the organism at the temperature 

 used in the experiment. The author has found this method inaccurate 

 by furnishing values which are too low, because the small quantity 

 of the original cultures transferred in pouring the Petri dishes may 

 not contain any live bacteria, though an appreciable number of 

 especially resistant individual organisms may be present in the whole 

 bulk of the bouillon. An absolutely trustworthy method is the 

 following : 



The tubes are heated and cooled as previously described, and then 

 the entire contents of each tube (for example, 5 or 10 c.c.), after 



