290 DETERMINATIONS OF THERMAL DEATH-TIME 



o.o per cent at 68.3° C. On retesting the 6.89 per cent, or 12 cultures, the following 

 variations were noted in a two-tube series heated at 62.8° C. for 30 minutes: 



Positive Positive 



First test 12 Fourth test 6 



Second test 4 Fifth test 9 



Third test 8 Sixth test o 



Thus the results show that cultures of B. coli heated for 30 minutes at 62.8° C. may or 

 may not survive, owing in all probability to the resistance of a few cells. Gage and 

 Stoughton' also noted that the temperature at which final sterilization occurred with 

 cultures of B. coli varied from 60° to 95° C. in 18 different tests, while the thermal 

 death-point of the majority lay between 50° and 55° C. in 14 tests and 55°-6o° C. in 

 4 tests. 



Any statement on the heat resistance of an organism must of necessity apply to 

 the strains tested, since it is possible that a study of a larger number of strains might 

 give entirely different results. It has been noted that sometimes conditions may de- 

 velop which affect the resistance of unknown types or even those already tested. 

 Variants may be produced under certain conditions of growth or storage, the spores 

 of which may exhibit varying resistance to heat. As to the comparative resistance of 

 types A and B, CI. bokdinum, Esty and Meyer^ found the average survival time of 

 the spores of 78 type A strains to be 40.1 minutes at 105° C, whereas the average 

 survival time of 30 type B strains at the same temperature was 23.7 minutes. Al- 

 though this shows that spores of type B are apparently less heat resistant than those 

 of type A, yet from the data presented in the same paper it is evident that under 

 exceptional circumstances strains and spores of a resistance equal to that recorded 

 for type A strains may be encountered in nature or may be produced artificially in the 

 laboratory. Dickson et al.^ also observed that in general the average resistance of 

 type A strains was somewhat higher than that of the type B strains, although the 

 maximum survival time of a type B strain (No. 10) when heated in oil-stratified broth 

 at 121° C. was found to be the same as that obtained in any of the tjqje A strains sub- 

 jected to the same temperature. He therefore concluded that many more tests must 

 be made before one may assume that type B strains of CI. botulinwn are consistently 

 less resistant to heat than type A strains. 



Not only the strains and the types but many other factors enter into the produc- 

 tion of resistant spores which may influence the resistance to heat. The composition 

 and the acidity of the media, the temperature at which the cultures are incubated, and 

 the anaerobic conditions employed are of primary importance. Attention has already 

 been directed to certain statements by Smith'' with respect to the sporulation of CI. 

 tetani. According to Esty and Meyer, ^ CI. bokdinum spores formed in certain media 

 were usually of a low resistance. In such cases sporulation was feeble. Von Hibler's^ 

 observations that "shyly sporulating" species in ill-adapted media produce spores 



' Gage, S., and Stoughton, G. V. E.: Technol. Quart., 19, 41. 1906. 



^ Esty, J. R., and Meyer, K. F.: loc. cit. 



3 Dickson, E. C, Burke, G. S., Beck, D., and Johnston, J.: loc. cit. 



■* Smith, T.: op. cit., p. 932. 1908. s von Hibler, E.: Joe. cit. 



