CHAPTER V. 



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 which as yet have not been cultivated outside the animal 

 body, and others, such as the gonococcus, which are in natural 

 conditions always parasites associated with disease. But these 

 latter 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. The 

 conditions of growth are, however, of very great importance in 

 the study of the 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-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 



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