CHAPTER V. 



RELATIONS OF BACTERIA TO DISEASE THE 

 PRODUCTION OF TOXINES 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 and the spirillum of relaps- 

 ing 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 methods 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 experi- 

 ment. 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 



