136 TEXT-BOOK OF BACTERIOLOGY. 



main point of the question. We may conveniently summarize our 

 opinions by saying: The pathogenic bacteria are those which, by 

 their vital action, produce excretions injurious to4he bodies of 

 men and animals j in the infectious species the excretions possess, 

 furtlier, tlie capability of overcoming the resistance offered to them 

 in the organism of susceptible animals, thereby enabling the bacteria 

 to multiply within the tissues. This capability may be lost if the 

 excretions alter their composition, which happens either where the 

 bacteria adopt a saprophytic mode of life or under the influence 

 of injurious agents affecting the cell-protoplasm. 



What conclusions may be drawn from these premises ? Let us 

 consider a few of them. 



In the course of this work we have often considered the differ- 

 ences of susceptibility in the same species of animals as regards 

 one and the same micro-organism. This explains itself now as a 

 differing amount of resisting power in the bodj' of the species in 

 question. Reference is here directed to the view taken when con- 

 sidering the attenuated anthrax bacilli, and it will be conceivable 

 that an insusceptible species of animal may be made susceptible bj' 

 a chemical influence affecting the tissue fluids. 



We owe to Behring a convincing proof of the correctness of this 

 assertion. Rats, as a rule, are not susceptible to anthrax infection, 

 probably because their blood possesses a particularly high degree 

 of alkalescence. If fed exclusively with vegetable nutriment, or if 

 acid phosphate of lime be added to their food (both being measures 

 by which the formation of acid in the organism is promoted), this 

 immunity disappears: the animals die when inoculated — i.e., an- 

 thrax bacilli have become infectious for rats so prepared for 

 them. 



Leo's discoveries will now be comprehended, and for many 

 varieties of susceptibility within one and the same species other- 

 wise inexplicable, we have here, it would seem, found the right ex- 

 planation. Thus it will be understood how, especially for the less 

 susceptible species, the quantity of infectious matter introduced 

 may be of importance. Let us adhere to our often-employed exam- 

 ple of the anthrax bacilli and suppose that some species of animal 

 which is insusceptible to them owes its immunity to a high degree 

 of alkalescence in its juices. A small number of anthrax bacilli 

 are not able to reduce the alkalescence sufficiently. If they are in- 

 troduced in increased numbers some indeed perish, but these dead 

 cells pour out their " acid " contents into the surrounding fluids and 

 open a possibility of life to the still living cells near them. Thus at 

 some one spot a field arises offering the conditions for bacterial de- 



