3O THE NATURE OF BACTERIA. 



tional conditions that large numbers of a single species are 

 found together, wholly unmixed with other species. Such 

 a condition, where great quantities of a single species of 

 bacterium are associated, uncontaminated with any individ- 

 uals of another species, is called by the bacteriologist a pure 

 culture. While such pure cultures are very unusual or al- 

 ' most never found in nature, they are easily enough produced 

 in the bacteriologist's laboratory by artificial methods. Pure 

 cultures have been coming into prominence in recent years, 

 and are to-day prepared by bacteriologists for various pur- 

 poses. They always represent artificial preparations and, 

 therefore, are usually unlike any natural conditions of bac- 

 terial life. 



BACTERIA AS AGENTS IN PRODUCING CHEMICAL 

 CHANGE. 



When we consider the extremely minute size of bacteria 

 it seems strange that they can be important agents in nature. 

 No single one can accomplish much, but their rapid mul- 

 tiplication gives them almost unlimited power. A possi- 

 bility of reproduction which will produce seventeen millions 

 of offspring in twenty- four hours is never actually realized, 

 but bacteria do multiply with inconceivable rapidity, and it 

 is this immense possibility of multiplication which makes 

 them agents of importance in nature. While growing and 

 multiplying so rapidly they are sure to produce profound 

 chemical changes in the food in which they live, and it is 

 these chemical changes produced in the medium surrounding 

 them that makes them agents of such significance. 



The chemical changes which are produced by bacteria 

 may be looked upon as of two general types : 



i. Synthetic Processes. These consist of chemical 

 changes by which complex bodies are built out of simpler 

 ones. Bacteria have a greater or less power of producing 

 such changes. For example, when the bacteria multiply 



