DEPARTMENT OF BACTERIOLOGY 161 



temperature is restored. As the temperature rises above one 

 iiundred it rapidly becomes unfavorable; different species have 

 different resisting power, but all or nearly all perish after a 

 few minutes' exposure somewhere between one hundred and 

 twenty and two hundred and twelve, the temperature of boil- 

 ing water. The spores which some species produce are much 

 more resistant than the bacteria themselves; this fact has to 

 be taken into account in using heat as a disinfectant. 



Reproduction. The bacteria reproduce in a very simple 

 way; a given microbe divides into two portions, each of which 

 becomes an independent bacterium; these two in their turn 

 produce two each, and so on indefinitely. When the condi- 

 tions are favorable reproduction is very rapid, so that in the 

 course of twenty-four liours the offspring of a single bacterium 

 may number millions. In addition to this method some bac- 

 teria form special reproductive bodies called spores, which 

 serve the same purpose as seeds in the higher plants. Spores 

 retain their vitality under conditions that would kill the mi- 

 crobes themselves. 



What is the role which bacteria play in nature? We laiow 

 that plants and animals are continually dying; we also know 

 that their bodies, after a longer or shorter period, disappear, 

 leaving behind only a small amount of matter which finally 

 becomes mixed with the soil, while the bulk of the substances 

 composing them disappears as gases. None of these materials 

 are lost, however, but somewhere and sometime they mav 

 and do enter into the composition of a new plant or animal. 

 The chief agents in the production of this decomposition or 

 decay of dead matter are the bacteria. They find in the dead 

 organic matter the materials which serve them as food and 

 when the other conditions are favorable they develop rapidly. 

 As a result of their activity the original compounds are broken 

 up and new ones are formed, usually simpler ones. These 

 various changes are spoken of as fermentations; when they are 

 accompanied by the production of foul odors, as in the decay 

 of animal substances, the process is termed putrefaction. 

 Some of the new substances are useful, others are harmless or 

 indifferent, while still others are violent poisons. 



