PAB. TELED, 
PATHOGENIC BACTERIA. 
L 
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 
microérganisms 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 
gonorrhcea, 
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 
