CHAPTER VI. 



RELATIONS OF BACTERIA TO DISEASE THE 

 PRODUCTION OF TOXINS BY BACTERIA. 



Introductory. It has already been stated that a strict division 

 of micro-organisms into saprophytes and true parasites cannot be 

 made. No doubt there are organisms, such as the bacillus of 

 leprosy, gonococcus, etc., which are in natural conditions always 

 parasites associated with disease. But these can lead a 

 saprophytic existence in specially prepared conditions, and there 

 are many of the disease-producing organisms, such as the 

 organisms of typhoid and cholera, which can nourish readily 

 outside the body, even in ordinary conditions. In fact we may 

 say that probably the cultivation of all pathogenic organisms on 

 artificial media is only a matter of time. The conditions of 

 growth are, however, of very great importance in the study of 

 modes of infection in the various diseases, though they do not 

 form the basis of a scientific division. 



A similar statement applies to the terms pathogenic and 

 saprophytic, and even to the terms pathogenic and non-patho- 

 genic. By the term pathogenic is meant the power which an 

 organism has of producing morbid changes or effects in the 

 animal body, either under natural conditions or in conditions 

 artificially arranged, as in direct experiment. Now we know of 

 no organisms which will in all circumstances produce disease in 

 all animals, and, on the other hand, many bacteria described as 

 harmless saprophytes will produce pathological changes if intro- 

 duced in sufficient quantity. When, therefore, we speak of a 

 pathogenic organism, the term is merely a relative one, and 

 indicates that in certain circumstances the organism will produce 

 disease, though in the science of human pathology it is often 

 used for convenience as implying that the organism produces 

 disease in man in natural conditions. 



Modifying Conditions. In studying the pathogenic effects in 

 any instance, both the micro-organisms and the animal affected 



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