CHAPTER VI. 



RELATIONS OF BACTERIA TO DISEASE— THE PRODUCTION 

 ■OF TOXINS BY BACTERIA. 



Introductory. — It has already been stated that a strict divi- 

 sion of micro-organisms into saprophytes and true parasites can- 

 not be made. No doubt there are organisms, such as the 

 bacillus of leprosy and the spirillum of relapsing fever, 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 flourish 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 pro- 

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

 often used for convenience as implying that the organism pro- 

 duces disease in man in natural conditions. 



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