250 PATHOGENIC BACTERIA IN MILK 



of the original suspension had been added. In all cases the 

 suspensions were injected directly into the peritoneal cavity of 

 guinea-pigs in order to give the bacilli the best opportunity for 

 multiplication. 



The experiments strengthened the supposition that the pellicle 

 which forms on milk in test-tubes during heating is responsible 

 for the increased resistance of milk suspensions of tubercle bacilli, 

 and the irregular results hitherto obtained. (This pellicle, which 

 is familiar to all who have scalded milk, is a feebly cohesive mass, 

 easily washed out in patches for microscopic examination by dilut- 

 ing the heated milk in water. The patches consist of fat globules 

 and an amorphous, cohesive substance of slight refrangibility, by 

 which they are held together.) 



The experiments demonstrate that tubercle bacilli are no more 

 resistant to heat than many other bacilli not producing spores, 

 and that at 60° C. destruction is complete in fifteen to twenty 

 minutes. Even after exposures lasting ten minutes the bacilli 

 were dead in most instances. After five minutes' exposures the 

 disease produced by inoculation in guinea-pigs was greatly 

 retarded, even though three times the control dose was injected. 

 When, however, milk is used as the suspending fluid, the forma- 

 tion of the surface pellicle into which bacilli are carried by fat 

 globules shields them from the effect of the heat, so that they may 

 survive an exposure of sixty-five minutes. The peculiarly irregular 

 results obtained by the Royal Commission are probably to be 

 explained in the same way. The importance of a clear under- 

 standing of this phenomenon in the pasteurisation of milk is 

 obvious, and it remains to be seen how far bottled milk may be 

 freed from tubercle bacilli without resorting to the higher tempera- 

 ture of 68° C. now generally employed. Probably a complete 

 immersion, or else a complete filling of the receptacle, may furnish 

 the conditions desired. 



The different results obtained by de Man, already quoted and 

 now generally accepted, are probably due to the fact that he used 

 caseous material or else tuberculous tissue, which he ground up 

 in a mortar and only diluted with salt solution " when necessary." 

 Such dense suspensions cannot be used to determine the thermal 

 death-point to be compared with that of other bacilli, nor can 

 they be regarded as imitating the conditions under which tubercle 

 bacilli appear in milk. 



The method of using narrow sealed tubes has shown itself 

 superior to that which employs ordinary test-tubes so far as milk 



