CHAPTER VIII. 

 CULTURES, AND THEIR STUDY. 



The objects which we have had before us in the prep- 

 aration of the culture-media were numerous. We have 

 prepared them so as to allow us to separate — or, rather, 

 to isolate — bacteria, to keep them in healthy growth for 

 considerable lengths of time, to enable us to observe their 

 biologic peculiarities, and to introduce them without dif- 

 ficulty into the bodies of animals. 



The isolation of bacteria was impossible until the fluid 

 culture-media of the early observers were replaced by the 

 solid media, and was exceedingly crude until Koch gave 

 us the solid, transparent media and the well-known 

 "plate-cultures." 



A growth of artificially-planted micro-organisms in 

 which an immense number are massed together is called 

 a culture. If such a growth contains but one kind of 

 organism, it is known as a pure culture. 



It has become the habit at present to use the term "cul- 

 ture" rather loosely, so that it does not always signify a 

 growth of micro-organisms artificially planted, but may 

 signify a growth taking place under natural conditions j 

 thus, typhoid bacilli are said to exist in the spleens of 

 patients dead of that disease "in pure culture," because 

 no other bacteria are there ; and sometimes, when in ex- 

 pectorated fragments of cheesy matter from tuberculosis 

 pulmonalis the tubercle bacilli are very numerous and 

 unmixed with other bacteria, the term "pure culture" 

 is again used to describe the condition. 



Three principal methods are at present employed to 

 enable us to secure pure cultures of bacteria, but before 

 beginning a description of them it is well to observe that 

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