80 JOAN F. POWELL 



ourselves what this heat resistance means to the organism in nature, i am 

 reluctant to believe these organisms are apt to be exposed in nature to the 

 temperatures we impose in the laboratory as the criterion for the physiologi- 

 cal nature of spores. One can only conclude we are using a technique which 

 admittedly is handy, but which nevertheless is quite beyond any plan that 

 nature has in mind producing spores. Resistance is an obvious selective 

 mechanism, to be sure, but it is difficult to visualize as a selective mechan- 

 ism resistance of the order of magnitude commonly and arbitrarily employed 

 in the laboratory. 



There are legions of microorganisms which contrive to survive in nature 

 by mechanisms clearly not involving either heat resistance or spore forma- 

 tion. Non-sporeforming bacteria have evolved in nature side by side with 

 sporeformers; consequently it is quite clear that spore resistance is not a 

 necessary requirement for survival. Are we truly characterizing the natural 

 physiological meaning of the spore by assigning resistance to it? In fungi 

 and in actinomycetes we have spore structures unquestionably analogous to 

 those in bacteria, yet their heat resistance comes nowhere near the order of 

 resistance typical of bacterial spores, at least as we evaluate them in the 

 laboratory. The temperature tolerance of fungal and actinomycetal condio- 

 spores by and large is only a few degrees above that of the corresponding 

 mycelium. Furthermore, there is some evidence in favor of the conclusion 

 that not all bacterial spores formed in nature are truly (heat) resistant. 

 Certainly, the loss of the remarkably high resistance which we set up as a 

 laboratory parameter of germination should not be taken as characteristic 

 of these spores in nature. 



Most of our laboratory cultures are highly selected populations, having 

 been cultivated on laboratory media, heat shocked, pasteurized, refrigerated, 

 and only God knows what else. Even in such spore types, highly selected 

 for resistance, we note an unmistakable tendency to generate spores which 

 do not possess the resistance characteristic of the other individuals in the 

 clone. It would be intriguing to have experiments that could shed light on 

 the problem of natural populations of spores (in soil, for example) and the 

 distribution of heat resistance. I dare say that a surprisingly large propor- 

 tion of the bacterial spores extant in nature, not being selected for heat 

 resistance, would fail the bacteriologist's test for a spore. Is it possible 

 that through our laboratory technology we have engendered a concept of a 

 monster spore instead of a typical spore? 



Among certain of the preceding speakers I noted differences of opinion 

 as to what characteristics, other than heat resistance, distinguish ungermi- 

 nated versus germinated spores. It occurred to me that an even more ele- 

 mentary question which would have to be answered satisfactorily is: what 



