74 BACTERIOLOGY. 



ing and thawing- tends to destroy bacteria. It is evident, 

 therefore that cold cannot be employed for the purpose of 

 destroying- bacteria. 



The action of heat is very different from that of cold. 

 Thus, many pathogenic bacteria when grown at 41 43 for 

 some time become so weakened that they are unable to 

 grow in the living- body. In other words, they become at- 

 tenuated. When this abnormal condition persists it eventu- 

 ally causes the death of the org-anism. 



The higher temperatures act in a similar way. They 

 first weaken and then destroy the organism. A tempera- 

 ture of 60 70 will destroy the ordinary vegetating bac- 

 teria in a few minutes. In such cases the heat probably 

 causes a coag-ulation of the protoplasm. As pointed out 

 on p. 56, spores may resist the action of steam-heat for 

 some minutes, and even for some hours, whereas the vege- 

 tating form is destroyed instantaneously. Heat coagula- 

 tion results in death because of the marked alteration in 

 the protein constituents of the cell. In cold coagulation, 

 on the other hand, the water within the cell is frozen but 

 the protoplasm is not altered. It is evident from what has 

 been said above, that cold acts as an antiseptic, whereas 

 heat is a germicide. 



When cultivating bacteria it is very important that the 

 temperature at which they develop shall be as nearly con- 

 stant as possible. In the case of pathogenic bacteria this 

 condition is usually observed, inasmuch as the organisms 

 are grown in an incubator which maintains a fairly constant 

 temperature. When saprophytic bacteria are cultivated, 

 or whenever g-elatin is employed, this is done at the so- 

 called room temperature, which may vary considerably. 

 At night it may be 10 or 15 below the temperature that 

 prevails during the day. The thermal oscillations of this 

 kind, alternately hastening and checking the growth of 

 bacteria, are not without effect upon the form of the organ- 

 isms, of the colonies and upon other cultural character- 



