130 Chapter VI 



medium and which are kept alive with very great difficulty outside 

 the liviug organism. 



What is to be said then of the vegetable micro-organisms which, in 

 [13S] this respect, are much less exacting? The most important of these 

 and the most numerous of all pathogenic micro-organisms, the 

 Bacteria, can as a rule be cultivated without difficulty not only in 

 the blood and fluids of animals that are susceptible or refractory 

 to their morbific action, but also on all kinds of vegetables and 

 artificial media: broths, fluids composed of mineral salts and of 

 certain organic substances. It is really not possible to attribute the 

 natural immunity of the dog and the fowl against the anthrax 

 bacillus — so fatal to a great number of mammals, man included, — 

 to its incapacity to feed on the fluids of these animals, when we see 

 that this same bacillus is capable of killing lower animals, such as 

 the cricket, and can thrive on carrots, potatoes and other vegetables. 



Even when, among the bacteria, we take those that are most 

 exacting in the choice of their food, we still find it impossible to 

 explain natural immunity as being due to the want of power on the 

 part of these organisms to obtain their nutriment from the juices 

 of refractory species. The bacillus discovered by R. Pfeiffer^ in 

 influenza does not develop on media that are ordinarily employed in 

 bacteriology in the cultivation of a great number of micro-organisms. 

 It needs a special food, which is prepared for it by spreading a drop 

 of fresh blood on the surface of agar. Pfeifier has established the 

 fact — confirmed by many observers — that the best species of blood 

 to use for this purpose is that of the pigeon. We should have to 

 believe, then, did the immunity really depend on the composition 

 of the fluids, that the pigeon is the least refractory of all animals. 

 Experiment has demonstrated the erroneousness of such a supposi- 

 tion: the pigeon is quite as refractory to Pfeiffer's bacillus as are 

 most other species of animals. 



As a second example the bacterium of bovine pleuro-pneumonia 

 may be cited. It is the smallest of all known bacteria. The diffi- 

 culties surrounding the discovery and identification of this organism 

 were very great, and the ingenuity of Nocard and Roux^ was required 

 for the demonstration of its existence. Very exacting in its choice of 

 nutritive material, it was first cultivated in the fluids of the rabbit,- 

 a si)ecies endowed with an absolute immunity against bovine pleuro- 



1 Ztschr.f. Hyg., Leipzig, 1893, Bd. xiii, S. 357. 

 * Ann. de VInst. Pasteur, Paris, 1898, t. xii, p. 240. 



