CHAPTER II 

 BIOLOGY OF MICRO-ORGANISMS 



The distribution of micro-organisms is well-nigh universal. They 

 and their spores pervade the atmosphere we breathe, the water we 

 drink, the food we eat, and luxuriate in the soil beneath our feet. 



They are not, however, ubiquitous, but correspond in distribution 

 with that of the matter upon which they live and the conditions 

 they can endure. Tyndall* found the atmosphere of high Alpine 

 altitudes free from them, and likewise that the glacier ice contained 

 none; but wherever man, animals, or plants live, die, and decom- 

 pose, they are sure to be. 



Their presence in the air generally depends upon their previous 

 existence in the soil, its pulverization, and distribution by currents 

 of the atmosphere. Koch has shown that the upper stratum of the 

 soil is exceedingly rich in bacteria, but that their numbers decrease 

 as the soil is penetrated, until below a depth of one meter there are 

 very few. Remembering that micro-organisms live chiefly upon 

 organic matter, this is readily understandable, as most of the organic 

 matter is upon the surface of the soil. Where, as in the case of 

 porous soil or the presence of cesspools and dung-heaps, the de- 

 composing materials are allowed to penetrate to a considerable 

 depth, micro-organisms may occur much farther below the surface; 

 yet they are rarely found at any great depth, because the majority 

 of them require free oxygen for successful existence. 



The water of stagnant pools always teems with micro-organisms; 

 that of deep wells rarely contains many unless it is polluted from the 

 surface of the earth. 



It has been suggested by Soyka that currents of air passing over 

 the surface of liquids might take up organisms, but, although he 

 seemed to show it experimentally, it is not generally believed. 

 Where bacteria are growing in colonies they seem to remain un- 

 disturbed by currents of air unless the surface of the colony becomes 

 roughened or broken. 



Most of the organisms carried about by the air are what are called 

 saprophytes, and are harmless. 



Oxygen. — As all micro-organisms must have oxygen in order to 

 live, the greater number of them grow best when freely exposed to 

 the air. Some will not grow at all where uncombined oxygen is 

 present, but secure all they need by severing it from its chemic 

 combinations. These peculiarities divide bacteria into the 

 * "Floating Matter in the Air." 

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