96 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 sepa- 

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

 earlier bacteriologists endeavored to separate bacteria of 

 different sorts by successive transplantations through a series 

 of tubes. The procedure now generally used for this pur- 

 pose is the so-called plate-method of Koch. The great 

 progress which bacteriology has made during the last twenty 

 years is largely owing to this invention. 



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 steam for a few minutes. 

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

 nus bacillus or hay bacillus, they may be expected to sur- 

 vive, and may perhaps be propagated in pure culture, 

 everything else having been killed by the steam. 



Plate-Cultures. It is impossible in most cases to dis- 

 tinguish between bacteria of different varieties by micro- 

 scopical 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 



