200 THE MILK QUESTION 



As a rule, bacteria are attenuated and lose their power 

 to infect before they lose their ability to vegetate upon 

 artificial culture media. It is, therefore, safe to assume 

 that a micro-organism that will not grow in artificial media 

 under favorable conditions is "dead." The tubercle ba- 

 cillus is an exception to this rule, for reasons given further 

 on. 



The methods used in the tests which I have made from 

 time to time were planned to imitate the actual conditions 

 of pasteurization, so far as practicable, in laboratory ex- 

 periments. 



The test tubes in which the infected milk was heated 

 were open to the air, and scum formation was disregarded 

 in all instances, my object being to determine the thermal 

 death-point against natural difficulties, so that the results 

 might be applied with confidence to practical pasteuriza- 

 tion. 



BaciUus tuberculosis. Certain special difficulties are met 

 with in determining the thermal death-point of the tubercle 

 bacillus. This organism does not grow readily upon artifi- 

 cial media. The few experiments made to determine its 

 thermal death-point by cultural methods have no signifi- 

 cance, because its vegetability upon artificial media does 

 not correspond to its power of growing in the animal or- 

 ganism. It is therefore necessary to inoculate animals in 

 order to determine whether or not the tubercle bacillus is 

 alive and virulent. Here again we meet with complications. 

 Dead tubercle bacilli have a certain amount of pathogenic 

 power and produce lesions, including tubercle formation, 

 abscesses, and coagulation necrosis. However, while we 

 lack a criterion to determine with precision the exact point 

 when the tubercle bacillus dies, we are able by means of 

 animal inoculations to determine just when the tubercle 

 bacillus is so enfeebled that it is no longer able to infect. 

 This, after all, is the important practical point. 



In my own experiments, in order to avoid the confusion 



