INFLUENCE OF VACUUM UPON GROWTH OF BACTERIA 285 



the laws of monomolecular reactions. According to this law, 

 if all spores were of equal resistance to heat, those present in the 

 largest number would be the last to disappear. However, spores 

 of different species of bacteria are not of equal resistance to heat, 

 and while those of a particular species may follow this law, it 

 cannot be appUed in a comparison of the thermal death rate of 

 different species. 



We have found that the several cultures of B. mesentericus with 

 which we have worked, are less resistant than the strains of B. 

 subtilis. Regardless of this decreased resistance B. mesentericus 

 predominated among the organisms isolated. Our cultures had 

 grown for some time upon culture media and the resistance may 

 have changed by this treatment. Weinzirl considers that B. 

 mesentericus predominates in canned foods because of its superior 

 heat-resisting quaUties. 



Lawrence and Ford (1916) state that the spores of B. sub- 

 tilis survive steaming one and one-fourth hours in the Arnold 

 sterilizer and autoclaving up to and including 19 pounds pressure 

 but are usually destroyed at 20 pounds. The B. mesentericus 

 spores survived one hour in the Arnold sterilizer and autoclaving 

 at 19 pounds pressure, being killed by 20 pounds pressure. These 

 statements would indicate that their cultures of B. subtilis were 

 somewhat more resistant than those of B. mesentericus. 



From some previous work upon these two types we had con- 

 sidered that B. mesentericus could grow in the absence of oxygen 

 more readily than B. subtilis. Cheyney in 1919, also calls at- 

 tention to this fact in his recent article on the bacteriology of 

 canned foods. 



We have occasionall}' isolated B. mesentericus from the deeper 

 layers of agar in our search for anaerobes, but we have never 

 isolated B. subtilis under such conditions, although it has been 

 found on the surface several times. It must be admitted that 

 we did not make quantitative determinations of the types present 

 in each jar. The predominance merely means that in the routine 

 isolation of colonies more of the B. mesentericus type were 

 isolated, although we are convinced that this organism did 

 predominate in the jars. 



