84 MANUAL OF BACTERIOLOGY. 



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 earher 

 bacteriologists endeavored to separate bacteria of different 

 sorts by 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 inocu- 

 lated with sputum containing tubercle bacilli mixed with other 

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

 may contain tubercle bacilli in pure culture, the other bacteria 

 having produced no important effect. 



Still another method which is occasionally useful is to subject 

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

 minutes. If it contains resistant spores, hke those of the tet- 

 anus 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 dis- 

 tinguish 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 



