272 PHYSICAL INFLUENCES 



BIOLOGICAL SIGNIFICANCE or THE CARDINAL POINTS OF TEMPERA- 

 TURE. The importance of the temperature requirements of certain 

 organisms to the role they play in nature can be illustrated by a few 

 examples. Most molds cannot cause disease in man and warm- 

 blooded animals because their maximum temperature is below the 

 body temperature. Exceptions are some Aspergilli and Mucorinece. 

 Pathogenic microorganisms must have their optimum temperature 

 coincide with that of their host. 



Organic substances may undergo a different change at different 

 temperatures. The biochemical changes in soil may not be the same 

 in northern Canada and near the Gulf of Mexico. Even the warm and 

 cold season of the same climate is apt to change not only the rate of 

 decomposition but possibly the products. Perhaps the most striking 

 example in this respect is the decomposition of ordinary market milk 

 kept at different temperatures. Such milk contains a great variety 

 of microorganisms; at various temperatures different types will pre- 

 dominate, while the remainder are retarded or inhibited by unfavor- 

 able temperature conditions and by the products of the dominant type 

 of bacteria. If milk is kept at about the freezing-point, only a few 

 organisms will develop slowly, but after a certain time their number 

 will increase to many million cells per c.c. There is, however, no appar- 

 ent change; no acid or deterioration can be discovered by the taste 

 though chemical analysis proves the presence of hydrogen sulphide 

 and ammonia. Between 15 and 25, milk will sour in about thirty- 

 six to forty-eight hours, giving a firm curd of an agreeable flavor 

 without whey or gas; later Oidium lactis destroying the acid develops 

 on the surface. Near body temperature the milk will lopper in twenty- 

 four hours, the curd is usually contracted, a large quantity of whey 

 is extruded, and much gas is produced by Bact. aerogenes and B. coll. 

 The odor is disagreeable and later butyric acid is produced; eventu- 

 ally the lactic acid increases further by the action of Bact. bulgaricum. 

 If kept above 50 the milk either keeps permanently, or a decomposi- 

 tion by thermophilic bacteria begins which is either an acid fermenta- 

 tion followed by digestion or a complete putrefaction, depending upon 

 the species of thermophilic organism that happens to be in the milk 

 sample. Thus there can be induced in the same substance, contain- 

 ing the same organisms at the start, four entirely different types 

 of decomposition merely by the difference of temperature. 



