[shanly] heat resistance OF BACTERIAL SPORES 123 



resistance between spores might well be a further means of differen- 

 tiation or diagnosis between species. 



It was with the two-fold object, first, of determining more pre- 

 cisely than hitherto the limits and the specific differences of heat resist- 

 ance in various bacterial endospores, and secondly, to determine 

 whether the evidence can be adduced which would allow us to regard 

 these bodies in the tubercle bacilli as spores, that the present inves- 

 tigation was entered upon. 



THE METHOD EMPLOYED. 



Previous experiments on the heat resistance of spores show that 

 there are many different factors influencing the thermal death point 

 of endospores which have to be taken into consideration. Dift"erences 

 in the constitution and chemical reaction of media, surface tension, 

 age and the previous history of cultures, and particularly the nature of 

 the heat employed, must all be taken into account in order to obtain 

 uniform results. The disturbing influence of these factors may, how- 

 ever, be counteracted to a large extent by the simple expedient of 

 subjecting all the cultures tested to an identical procedure, both as 

 regards preliminary growth and age of growth, and as regards medium 

 and mode of testing. Our object was not to study these factors of 

 variation, but on the contrary by subjecting all the various cultures 

 to the same procedure to gain thus constants which would permit us 

 to answer surely the main question: do different species of spore- 

 bearing bacteria present constant differences in the susceptibility of 

 their spores to the action of moist heat ? 



Preliminary observations demonstrated that surface cultures 

 upon agar-agar media of the various forms employed showed no ob- 

 vious alteration in the resistance of the spores between a week and 

 two months. Such surface cultures were therefore employed through- 

 out, and for each test a suspension of the spores and spore-bearing 

 bacteria taken from an agar culture at least a week old was used. 

 The medium of suspension was standard "Lemco" peptone bouillon 

 rendered one per cent acid to Phenolphthalein. That they might 

 heat up rapidly, small, thin-walled test tubes 0.5 mm. thick and 12 

 mm. in diameter, were half filled with the broth and brought to the 

 required heat in the water bath about to be described ; when the proper 

 temperature was attained the spores were introduced by means of a 

 platinum loop. Our early experiments had impressed us with the 

 difficulty of maintaining a constant temperature, and more particularly 

 with the slow arrival of the fluid within a test tube at the temperature 

 of the surrounding fluid. For this reason it has seemed to us that the 

 routine exposure of spores to a stated temperature for ten minutes or 



