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 0° 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 
