RESISTANCE OF TUBERCLE BACILLUS 251 



suspensions are concerned. When other fluids were used the 

 results were unequivocal and in harmony with the other method, 

 excepting perhaps in the last experiment, in which an increased 

 resistance of the tubercle bacilli in water is manifest. 



The two chief conclusions drawn by Theobald Smith from his 

 experiments are : — 



1. Tubercle bacilli when suspended in distilled water, normal 

 salt solution, bouillon, and milk are destroyed at 60' C in fifteen to 

 twenty minutes. The larger number are destroyed in five to ten 

 minutes. 



2. When tubercle bacilli are suspended in milk, the pellicle 

 which forms during the exposure at 60' C. may contain living 

 bacilli after sixty minutes. 



The last experiments in relation to thermal death-point which 

 need be quoted are those of Russell and Hastings.^ This series 

 of experiments was of a very thorough character. 



The standards previously used in pasteurisation had been 150' 

 to 155' F. (65'' to 68° C), for fifteen to twenty minutes. Under 

 these conditions the consistency of milk is materially changed. 

 The cream does not rise readily in milk so treated, and the con- 

 sistency or " body " of cream itself is much lessened. If the 

 exposure is made at the temperature of 140'^ F. (60' C.) or below, 

 this diminution in consistency does not occur. In fact milk or 

 cream heated no higher than this cannot be distinguished from the 

 normal product. Hence these experiments were undertaken, in the 

 first instance, to check the work of Theobald Smith quoted above. 



If it is possible effectually to destroy the tubercle bacillus at 

 this temperature (140"^ F.) by an exposure for a brief period of 

 time, a new standard for pasteurisation may be made, that will 

 enable one to overcome the only objection that has been raised 

 against the use of this product. 



The results of the experiments of Russell and Hastings are as 

 follows : — 



1. An exposure of tuberculous milk to 140° F. (60"* C.) in a 

 tightly closed commercial pasteuriser for a period of ten minutes, 

 destroyed in every case the tubercle bacillus as determined b}- 

 the inoculation of such heated milk into susceptible animals like 

 guinea-pigs. 



2. Where milk was exposed under conditions that would en- 

 able a pellicle or membrane to form on the surface, the tubercle 



^ Seventeenth Annual Report 0/ the Wisconsin Agricultural Expt. Sta., 1900, 

 pp. 147-170. 



