CHAPTER VIII. 

 CULTURES, AND THEIR STUDY. 



THE purposes for which culture media are prepared are 

 numerous. Through their aid it is possible to separate 

 or, rather, to isolate bacteria, to keep them in healthy 

 growth for considerable lengths of time, during which their 

 biologic peculiarities can be observed and their metabolic 

 products collected, and to introduce them free from con- 

 tamination into the bodies of experiment animals. 



The isolation of bacteria was next to impossible until the 

 fluid media of the early observers were replaced by the solid 

 culture media introduced by Koch, and exceedingly diffi- 

 cult until he devised the well-known "plate cultures." 



A growth of artificially planted micro-organisms is called 

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

 ism, it is known as a pure culture. 



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

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

 artificially planted growth of micro-organisms, but may 

 signify a growth taking place under natural conditions ; thus, 

 typhoid bacilli are said to occur in " pure culture" in the 

 spleens of patients dead of that disease, because no other 

 bacteria are associated with them ; and sometimes, when the 

 tubercle bacilli are very numerous and unmixed with other 

 bacteria, in the expectorated fragments of cheesy matter 

 from tuberculosis pulmonalis, they are said to occur in 

 "pure culture." 



The culture manipulations are performed either with a 

 sterilized platinum wire or with a capillary pipet of glass. 



The platinum wire is so limber that it is scarcely to be 

 recommended, and a wire composed of platinum and iridium, 

 which is elastic in quality, is to be preferred. The wires 

 are about 5 cm. in length, of various thicknesses according to 

 the use for which they are employed, and are usually fused 

 into a thin glass rod about 17 cm. in length (Fig. 37)- The 

 wires may be straight or provided with a small loop at the 

 end so as to conveniently take up small drops of fluid. 

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