CHAPTER V. 

 CULTIVATION OF BACTERIA, CONTINUED. 



Isolation of Bacteria. — In order to study any kind of 

 bacteria it is necessary to have the particular species separated 

 from other sorts with which it may be mixed. The earlier 

 bacteriologists endeavored to separate bacteria of different 

 sorts of successive transplantations through a series of tubes. 

 The procedure now generally used for this purpose is the so- 

 called plate-method of Koch. The great progress which 

 bacteriology has made during the last twenty years is largely 

 owing to the use of this method. 



Pathogenic bacteria may sometimes be isolated through 

 inoculations into animals. Thus an animal may be inoculated 

 with sputum containing tubercle bacilli mixed with other bac- 

 teria. The animal may die of tuberculosis, and its tissues may 

 contain tubercle bacilli in pure culture, the other bacteria hav- 

 ing produced no important effect. 



Still another method which is occasionally useful is to sub- 

 ject the mixture of bacteria to a heat of 80'' or 90° C. for a few 

 minutes. If it contains resistant spores, like those of the teta- 

 nus bacillus or hay bacillus, they may be expected to survive, 

 and may be propagated in pure culture, everything else having 

 been killed by the heat. 



Plate-cultures. — It is impossible in most cases to distin- 

 guish between bacteria of different varieties by microscopic 

 examination alone. Bacteria of widely different species and 

 quite unlike one another in their properties may present 

 similar appearances under the microscope. The differences 

 which they exhibit are usually apparent when they are grown 



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