38 Bacteria in Relation to Country Life 



characteristic chemical substances, such as pigments, 

 enzymes, and toxins. Heating within certain limits 

 may, therefore, serve as a means for the production of 

 so-called attenuated or weakened cultures. These 

 may also be produced by other methods, particularly 

 by the limited action on the organisms of germicides, 

 as corrosive sublimate, carbolic acid, and chloroform. 



Moisture content of the culture medium. — Bacteria 

 growing in solid substances will discontinue their growth 

 when the proportion of moisture in the medium reaches 

 a certain minimum. Studies on the decay of humus in 

 the soil have shown that the decomposition processes 

 are practically at a standstill with 2 to 3 per cent of 

 moisture; that with 4 to 5 per cent they are more active; 

 and that they finally reach a maximum beyond 25 to 

 30 per cent of moisture. In vegetable and animal sub- 

 stances, the minimum amount of moisture required 

 for the development of bacteria is much higher, scarcely 

 any growth occurring when the moisture content is 

 less than 25 per cent. 



Relation to oxygen. — Bacteria show widely different 

 relations in their behavior towards the oxygen of the 

 air. Some species will not develop at all when air is 

 excluded or its supply limited beyond a certain point. 

 These organisms are designated as aerobes. Many of 

 the most common soil and water bacteria are strict 

 aerobes, or obligate aerobes (to use a term in vogue 

 among bacteriologists), and they include decay bacteria, 

 nitrifying bacteria, and nitrogen-fixing bacteria. 



There are, on the contrary, numerous other species 

 that require the entire exclusion of air for their proper 



