330 AGRICULTURAL BACTERIOLOGY. 



The bacteria which live as parasites in domestic animals con- 

 stitute one of the most serious evils which the farmer has to 

 meet. Fortunately the number of species of bacteria capable 

 of living a parasitic life is small. Nearly all types of bacteria 

 which we have hitherto considered are utterly unable to live 

 the life of parasites, and are, therefore, so far as their power of 

 doing direct injury to animals is concerned, entirely harmless. 

 Of the many hundreds, and probably thousands, of species of 

 bacteria known to-day, only a very small number, a score or 

 two, are positively known to be able to produce disease, and 

 these organisms are commonly found only in special places 

 and not living a free life in nature. With very few exceptions 

 the bacteria which do live a free life in nature are utterly 

 harmless because of their inability to live a parasitic life. The 

 great importance of this conclusion is evident. 



RESISTANCE AGAINST PATHOGENIC BACTERIA. 



An extremely important topic for consideration centers 

 around the question why the great hosts of harmless bac- 

 teria are unable to live a parasitic life. The answer, in brief, 

 is that the active tissues of animals and plants in some way 

 exert an influence upon the bacteria which prevents their 

 growing. For example : the putrefactive bacteria arc able to 

 grow with the greatest readiness in a piece of dead flesh from 

 a cow, causing its rapid disintegration. But if these same 

 bacteria are inoculated into the blood of a living cow, or into 

 its flesh, they are utterly unable to live for any length of time, 

 and will, under ordinary conditions, die very speedily. Why 

 should living flesh exert such a deleterious influence upon 

 bacteria, while the same flesh, after death, offers such a favor- 

 able food for them ? The complete answer to this question is 

 one for which bacteriologists have been searching for some 

 years and which they have only partly reached. This matter 

 does not particularly concern us and we need only notice that 



