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 true parasites and saprophytes cannot be 

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

 tubercle, 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 flourish readily 

 outside the body, even in ordinary conditions. A similar state- 

 ment applies to the terms pathogenic and non-pathogenic. 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 organ- 

 isms 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 

 must be considered, and not only the species of each, but also 

 its exact condition at the time of infection. In other words, 

 the resulting disease is the product of the sum-total of the 

 characters of the infecting agent, on the one hand, and of the 

 subject of infection, on the other. We may, therefore, state 

 some of the chief circumstances which modify each f of r these 



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