INFLUENCE OF VACUUM UPON GROWTH OF BACTERIA 293 



decrease in case of B. subtilis. Parallel tubes which were not 

 scaled showed heavy growth of these organisms in all concentra- 

 tions of salt, but onlj' a trace of growth in the higher amounts of 

 acid. In the sealed tubes there was a very faint visible trace of 

 surface growth in the tubes containing larger amounts of air. 

 This was easily broken up and did not re-form on standing. The 

 plates made from these tubes checked much better than would 

 generally be expected, when it is considered that organisms of 

 this type produce such heavy surface growths in open tubes. 

 However, these organisms do not form such adherent growths in 

 sealed tubes, and by vigorous shaking a fairly uniform suspension 

 may be obtained. 



In the case of B. mesentericus there was much less marked 

 action of acid. Either there is not so much decrease in the higher 

 amounts of acid or there is a sUght increase of this organism 

 after the initial decrease. At the end of the incubation period 

 there were about the same numbers as at the beginning of the 

 experiment. 



In Experiment /F an attempt was made to determine the in- 

 fluence of the amount of air and salt upon the thermal death 

 point of B. subtilis and B. mesentericus. The results are shown in 

 tables 6 and 7. The tubes and spore suspensions were prepared 

 as above. 



It is evident from these tables that B. mesentericus spores are 

 somewhat more casly killed by heat than those of B. subtilis. 

 We have found this to be true in numerous other tests upon the 

 thermal death point of these organisms. 



The amount of salt or the amount of air has practically no 

 influence upon the thermal death point, particularly in the case 

 of B. subtilis, the larger amounts of salt seeming, however, to 

 protect the organisms to some extent. 



Experiment V shows the influence of acetic acid and varying 

 amounts of air upon the thermal death point of B. subtilis and 

 B. mesentericus. 



In this experiment the spores were prepared as above described. 

 The Uquid in which the spores were suspended during the heating 

 was extract broth plus 0.5 percent; 1 per cent; 2 per cent; 4 per 



