Conditions Favoring Growth of Bacteria 39 



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 decomposing materials are allowed 

 to penetrate to a considerable depth, the bacteria 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 bacteria, 

 but that of deep wells rarely contains many unless it is 

 polluted from the surface of the earth. 



Being generally present in the soil, which the feet of men 

 and animals grind to powder, the bacteria, together with the 

 pulverized earth, are blown from place to place into every 

 nook and cranny, until it is impossible to escape them. It 

 has been suggested by Soyka that currents of air passing 

 over the surface of liquids might take up bacteria, but, 

 although he seemed to show it experimentally, it is not 

 generally believed. Where bacteria are growing in colonies 

 they seem to remain undisturbed by currents of air unless 

 the surface of the colony becomes roughened or broken. 



Most of the bacteria that are carried about by the air 

 are what are called saprophytes, and are perfectly harm- 

 less to the human being; but not all belong to this class, 

 nor will they do so while tuberculous patients are allowed 

 to expectorate upon the sidewalks, and typhoid patients' 

 wash to dry upon the clothes-line, and their dejecta to 

 be spread upon the ground. 



The growth of bacteria is profoundly influenced by 

 environment, so that a consideration of the conditions 

 favorable and unfavorable to their existence becomes 

 necessary. 



Conditions Favoring the Growth of Bacteria. 

 (a) 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 



Aerobes, which grow in the presence of uncombined 

 oxygen, and the 



Anaerobes, which do not grow in the presence of un- 

 combined oxygen. 



