256 BACTERIOLOGY. 



may be supplanted or outgrown by other organisms — second- 

 ary infection. 



The value of artificial culture, in suspected cases of 

 disease, is seen in diphtheria. On the other hand, the pus, 

 sputum, or pleuritic fluid from a diseased person may not 

 show, on direct microscopic examination, any sign of the 

 tubercle bacillus. Yet, if such material is injected into a 

 guinea-pig, it may produce tuberculosis and death. The 

 microscope failed to detect the few bacilli present but the 

 living body afforded these same cells an excellent medium 

 for growth, and^ as a result, they rapidly multiplied and 

 eventually brought about the death of that animal. An 

 examination of the altered tissue, in such an animal, will 

 reveal the presence of myriads of tubercle bacilli. Since 

 living cells are derived only from cells of their own kind, 

 it follows that these organisms were present in the original 

 diseased condition. 



The mere fact that an organism is constantly present 

 in a given disease, does not prove that it is the cause. 

 It certainly is strong presumptive evidence that the 

 organism does bear a causal relation, but at the same 

 time the possibility must be admitted that it may be an 

 accompaniment, or even a consequent of that disease. 

 The latter view, it is scarcely necessary to note, would im- 

 ply the existence of spontaneous generation. To complete 

 the chain of evidence, it is necessary, therefore, to obtain 

 the organism in an absolutely pure culture, free from any 

 other organism or foreign chemical poison. Subsequent 

 inoculation of susceptible animals with such cultures must 

 reproduce the disease. Whenever possible, an animal 

 should be selected which is naturally susceptible to the 

 disease. An animal that is not subject to the disease can 

 hardly be expected to reproduce the typical affection on in- 

 oculation with the organism. 



The isolation of the organism and the preparation of 

 pure cultures, is accomplished by means of the gelatin 



