LESSON X. 



STUDY OF THE PATHOGENIC ACTION OF BACTERIA. 



While all bacteria are necessarily of general interest as being either indirectly 

 or directly important to our life and relations, those which produce disease in man 

 are of essential interest to medical men. -To these the name pathogenic bacteria is 

 commonly applied, the remainder being termed non-pathogenic bacteria. The known 

 pathogens form a comparatively small proportion of the numerous species thus 

 far discovered; but it should not be forgotten that we have little idea of the possi- 

 bility of pathogenic power which under even apparently unimportant modifica- 

 tion of condition may reside in the so-called non-pathogens. Anthrax is ordinarily 

 non-pathogenic to chickens, whose body temperature is comparatively high; but 

 should the hen's body heat be lowered several degrees by refrigeration, she becomes 

 susceptible to the influences of the bacterium of this disease. The relative amount 

 of acid or alkaline substances which Microspira comma meets in the alimentary canal 

 of man apparently determines whether he shall escape or be stricken with Asiatic 

 cholera; and Bacillus coli, a constant and profuse parasite of man's intestinal canal, 

 is often met in other situations in the body, apparently the cause of serious lesions. 

 It is not impossible that every bacterium may somewhere in the range of higher life, 

 animal or vegetable, find itself capable of a parasitic existence; such parasitism may 

 doubtless be forced by transient and perhaps unobserved special circumstances or 

 conditions. Parasitism does not necessarily imply pathogenesis ; but should it happen 

 that the host is susceptible to the various influences exerted by the parasite and its 

 products, then disease must arise. It is from such a view not a wonderful thing that 

 diseases unusual to man should occasionally appear, their entrance and development 

 depending upon some accident or some temporary and unusual condition; nor can 

 it be looked on as impossible that totally new disease occurrence may take place, 

 depending upon a forced invasion of some microbe hitherto unknown as a human 

 parasite and its development favored by some unusual condition which may exist 

 in the body of the affected individual. Correlatively, while we are accustomed to 

 speak of certain microbes as being obligate parasites, this is probably only in a general 

 sense correct; in strict meaning it can scarcely be doubted that under unknown but 

 definite conditions the parasites of man might be reduced to parasitism in some other 

 species or even to a saprophytic existence. Thus is explained the fact that human 

 disease of some sort may sometimes apparently die out and be unknown for a greater 

 or less period, returning, when conditions favor, to the surprise and consternation 

 of men. 



It is well, therefore, to think of all bacteria as having a possible pathogenic in- 

 fluence sometimes and to some species of created beings; and no matter what the 

 source of the microorganism studied, a part of that study forced upon us for our own 

 protection must concern itself with the influence the bacterium may exert upon higher 

 life. It is impossible, of course, that its relations to all forms of life be investigated ; 



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