PART THIRD. 



PATHOGENIC BACTERIA. 



I. 



MODES OF ACTION. 



MANY of the saprophytic bacteria are pathogenic for man, or for 

 one or more species of the lower animals, when by accident or ex- 

 perimental inoculation they obtain access to the body ; these may be 

 designated facultative parasites. Other species which, for a time 

 at least, are able to lead a saprophytic mode of life have their nor- 

 mal habitat in the bodies of infected animals, in which they produce 

 specific infectious diseases. To this class belong the cholera spirillum, 

 the anthrax bacillus, the bacillus of typhoid fever, and various other 

 microorganisms which are the cause of specific infectious diseases in 

 some of the lower animals. These we may speak of as parasites 

 and facultative saprophytes. Still others are strict parasites and 

 do not find the conditions for their development outside of the bodies 

 of the animals which they infest, except under the special conditions 

 in which bacteriologists have succeeded in cultivating some of them. 

 The best known strict parasites are the tubercle bacillus, the bacillus 

 of leprosy, the spirillum of relapsing fever, and the micrococcus of 

 gonorrhosa. 



There can be but little doubt that even the strict parasites, at some 

 time in the past, were also saprophytes, and that the adaptation to a 

 parasitic mode of life was gradually effected under the laws of natural 

 selection. In a previous chapter (Section III., Part Second) we have 

 referred to the modifications in biological characters which may 

 occur as a result of special conditions of environment. Thus we may 

 obtain non-chromogenic varieties of species which usually produce 

 pigment, or non-pathogenic varieties of bacteria which are usually 

 pathogenic. There is also evidence that the tubercle bacillus, a strict 



