[shanly] heat resistance OF BACTERIAL SPORES 131 



DISCUSSION OF CHART. 



Nevertheless attention must be called to certain veiy impoitant 

 points that have presented themselves during the course of this 

 investigation. These I will take up in detail. 



1. It is obvious that all endospores do not possess the same re- 

 sisting power to heat. 



2. Very few spore-bearing species possess spores which are 

 capable of surviving a moist heat of 100 °C. for sixty minutes. Only 

 two of the series possess this power. 



3. B. Anthracis, which is so often spoken of as the type resistant 

 sporulating organism, will only survive a temperature of 85 °C. for 

 one hour, thus coming relatively low down in the list. The strain 

 employed by us was an old stock laboratory strain which has been in 

 the Pathological Laboratory of McGill for many years. 



4. At the other extremity of the scale is a culture of B. cereus 

 obtained from Dr. Winslow which failed to survive exposure to 

 75 °C. for one hour. This form possesses what otherwise were typical 

 endospores.^ 



5. So far as it is safe to draw any conclusions from this restricted 

 series, an hour's exposure to a temperature of 70°, rather than 80°C. 

 affords the test for the existence of endospores. 



6. While this is the case, it has been most noticeable that tem- 

 peratures here given are not the temperatures resisted by all the spores 

 of a given culture ; on the contrary the majority of the spores are killed 

 by the standard exposure to temperatures 10-20 degrees below those 

 here indicated. Or in other words, the highest positive sign in the 

 chart represents the upper limit of heat resistance possessed by a 

 minority of the spores of any given culture in our series: it represents 

 the maximal resistant power of the spores exhibited by some only of the 

 endospores present in a given culture of a given strain of a bacterial 

 species. The heat resistance of spores is thus very far from being a con- 

 stant quantity, even if possibly in a given species different cultures 

 have this in common, that they will afford a certain number of spores 

 having the same maximal thermal death point. 



We do not believe that this is generally recognized. The follow- 

 ing experiment is one of a large number which we have undertaken. 

 Two sets of agar tubes were melted by boiling, then the first was placed 

 in the water-bath and allowed to come to 90 °C., the second was cooled 

 in water to about 40 °C. The culture of B. Anthracis and six cultures 



^ Note during proof reading. On repeating our tests with transplants from the 

 original cultures, I find that the spores of B. cereus now withstand treating to 80°C. 

 for one hour. So also I obtain positive results with B. vulgatusand B. mesentericus at 

 90°C. 



