248 



PATHOGENIC BACTERIA IN MILK 



milk question indirectly. Mention will also be made of the effect 

 of other physical agencies. 



(i.) High temperatures.— A large number of experiments have 

 been made, with the object of establishing the thermal death-point 

 of the B. tuberculosis. There has been considerable discrepancy in 

 the results that have been obtained. The reason for this is due in 

 part to the varied degrees of virulence of different strains of 

 tubercle bacillus, and in part to the fact that there has been little 

 or no uniformity in the carrying-out of the experiments. In some 

 instances tuberculous tissue has been used, in others cheesy pus, 

 in others pure cultures of more or less lengthened artificial cultiva- 

 tion, and, in others, sputum. Again, different media have been 

 used in which to test the thermal death-point. The difficulty of 

 growing the bacillus on artificial media may also have affected the 

 experiments in some cases. 



As far back as 1884 Schill and Fischer made a number of 

 experiments to determine the thermal death-point, and they found 

 that an exposure at 100° C. for five minutes destroyed the tubercle 

 bacillus in sputum. Three years later Sternberg inoculated 

 guinea-pigs with tuberculous sputum subjected for ten minutes to 

 the following temperatures, 50°, 60'', 70°, 80°, 90° C. That exposed 

 at and above 60° C. was rendered innocuous. Yersin found a 

 temperature between 65° and 70° C. for ten minutes to be the death- 

 point.^ Many other workers obtained somewhat similar results 

 (Grancher, Lebard, Bonhoff, Beck, Hewlett, etc.). 



In 1893 de Man,2 working with broken-down, semi-fluid cheesy 

 matter derived from tuberculous udders, obtained the following 

 results, which have been generally accepted as the standard quoted 

 in text-books : — 



' Ann. de I Inst. Pasteur, 1888, ii., 60. 

 '^ Archiv. fur Hygiene, 1893, xviii., 133. 



