TEXT-BOOK OF BACTERIOLOGY. 65 



of diseases in general. Here far more than in otlier spheres is to 

 be found in the recognition of its true cause the key to a right un- 

 derstanding of all the different symptoms under which a disease 

 appears. 



I. STERILIZATION. 



To obtain the most advantages possible from the artificial breed- 

 ing of bacteria, certain precautionary measures must be employed. 



To get a clear, definite idea of the peculiar properties and gen- 

 eral behavior of a particular m'icro-organism, it is above all neces- 

 sary to bring that particular species under observation alone and 

 free from all admixture with other species. A medley of bacteria 

 is useless for exact investigations. It is only when one species is 

 obtained in pure culture, as it is called, that it is possible to rely on 

 obtaining safe, unobjectionable results. 



The characteristic marks of a species which are perhaps too 

 slight in the individual to be easily noticeable are in cultivation 

 made clearly evident, and all that was characteristic of the species 

 in question now shows itself in a greater degree witii numberless 

 repetitions. Attention was earlj^ directed to these advantages, and 

 investigators endeavored to find some means of obtaining pure 

 cultures of the various species of bacteria. But it was soon found 

 (the more so that ail attempts were made with liquid media) that 

 the problem presented great difficulties — difficulties arising from 

 the immense diffusion, it might be said the omnipresence, of the 

 bacteria and the high power of resistance of their spores. 



Of course, in order to breed a particular species of bacterias- 

 artificially in pure culture on any nutrient medium, the latter must 

 first be freed from all other micro-organisms before employing it,, 

 and further, every pure culture must be protected during its growth 

 from the invasion by foreign bacterial germs — i.e., from contami- 

 nation. 



And this is no easy task, especially the first destruction of 

 germs in the culture media — their "sterilization," as the French 

 call it and as it ,is now universally called — requires particular 

 attention. 



The bacteria in their common form are not particularly able to 

 withstand exterior influences. A number of them are, however, 

 provided with a peculiar protective arrangement, destined to pre- 

 serve the species in the face of hostile exterior influences, and, as 

 already mentioned, the fruits or spores which serve this purpose 

 are perhaps the most tenacious of life among all the organized 

 beings of our world. To free a nourishing solution or anj^thing 

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