TEMPERATURE 41 
at which the protoplasm of most animals and plants is coagulated. 
The vast majority of bacteria, however, develop best within a range 
of temperature from 15° C. as a minimum to 40 to 43° C. as a maxi- 
mum. All bacteria exhibit three cardinal thermic points: a minimum 
temperature, below which growth ceases; an optimum temperature, 
at which growth is most luxuriant and rapid; and a maximum tem- 
peratme, above which growth ceases and the organisms die. Fischer^ 
has classified bacteria according to their thermic relations as follows: 
Minimum. Optimum. Maximum. 
1. Psychrophilic bacteria . 15-20 30 Many water bacteria. 
2. Mesophilic bacteria 15-25 37 43 Pathogenic bacteria and 
others. 
3. Thermophilic bacteria . 25-45 50-55 85 Spore-forming bacteria 
from soil, feces and 
thermal springs.^ 
Bacteria which are progressively pathogenic for man and warm- 
blooded animals develop within a much narrower range of tempera- 
ture than the saprophytic bacteria which are found chiefly in Nature, 
as the following table, also taken from Fisher,^ indicates: 
Difference between 
minimum and 
Minimum. Optimum. Maximum. maximum. 
B. phosphorescens .... 20 38 38 
B. fluorescens 5 20-25 38 33 
B. subtilis 6 30 50 44 
Vibrio cholera; 10 37 40 30 
B. anthracis 12 37 45 33 
B. diphtheria; 18 33-37 45 27 
Mic. gonorrhese .... 25 37 39 14 
B. tuberculosis 30 37 42 12 
B. thermophilus .... 40 60 80 40 
The saprophytic bacteria, as for example B. subtilis, which develop 
through a relatively wide range of temperature are also called Eury- 
thermic bacteria. The pathogenic bacteria, as for example, the tubercle 
bacillus, which exliibit but little latitude in this respect, are called 
Sthenothermic bacteria. 
2. Cold. — All bacteria grow best and most rapidly in an environment 
which is maintained at the optimum temperature for the organism. 
If this temperature is lowered even a few degrees, the rate of reproduc- 
tion is proportionately reduced. As the temperature approaches 0° C, 
there is complete or nearly complete cessation of growth with a corre- 
sponding complete or nearly complete restriction of chemical inter- 
change between the organism and its environment. The viability, 
and in the pathogenic bacteria the virulence, is not seriously impaired 
even by exposure to these low temperatures for considerable periods 
of time. Practical advantage is taken of this restriction of bacterial 
1 Vorlesungen liber Bakterien, 1903, II. Aufl. 
2 See Morrison and Tanner: Jour. Bacteriol., 1922, 7, 343. 
' Loc. cit., p. 106. 
