146 BACTERIOLOGY. 



gradually acclimated anthrax to a temperature of 

 12 C., when it killed frogs kept at 12 C. We have 

 cultivated a very virulent diphtheria bacillus, so that 

 it will grow at 43 C. and produce strong toxin. 



Bacterial growth is retarded by temperatures only a 

 little below the minimum of the species in question; 

 but they are not otherwise injured. Indeed, it is the 

 usual custom in laboratories to preserve bacteria 

 which die readily (such as streptococci) by keeping 

 them in the refrigerator at about 4 to 6 C , after cul- 

 tivation for two days at 20 C., as a means for retain- 

 ing their vitality without repeated transplantation. 

 Temperatures even far under* C. are only slowly 

 injurious to bacteria, different species being affected 

 with varying rapidity. Ordinarily, low temperatures, 

 though arresting the growth, do not destroy the vitality 

 of bacteria. This has been demonstrated by numerous 

 experiments in which they have been exposed for hours 

 in a refrigerating mixture at 18 C. They have 

 even been subjected by us to a temperature of 175 

 C. by immersing them in liquid air kept in an open 

 tube for two hours, and found to grow still when placed 

 in favorable conditions. 



Temperatures from 5 to 10 C. over the optimum 

 affect bacteria injuriously in several respects. Varieties 

 are produced of diminished activity of growth, the 

 virulence and the property of causing fermentation are 

 decreased, and the power of spore-formation is gradu- 

 ally lost. These effects may predominate either in one 

 or the other direction. 



If the maximum temperature is exceeded the organism 

 dies; the thermal death- point for the psychrophilic spe- 

 cies being about 37 C., for the mesophilic species about 



