408 HUMAN BIOLOGY 



than in other living forms, because the artificial production of 

 changes or mutations, both as to growth characteristics 

 and infectious power, can be easily and rapidly accomphshed 

 in the laboratory. 



As a matter of actual observation we may subdivide 

 bacteria into definite classes according to the degree to 

 which they developed parasitic properties, as follows: 



1. Those which under no circumstances will grow, except in the 

 saprophytic state, upon dead organic and inorganic materials. 



2. Those which have become adapted to an environment 

 supplied on the physiologic exterior of the bodies of other forms 

 where the reaction and the substance of secretions and waste 

 products supply them with a suitable environment. 



3. Those which, living on the exterior of the body, may produce 

 powerful poisons by which the host is injured both locally and 

 generally. To this class, which still may be regarded as purely 

 saprophytic, belong some of our important pathogenic organisms, 

 the diphtheria and the tetanus bacilli, which do not actually 

 enter the tissues and may therefore be spoken of accurately as 

 pathogenic saprophytes. Unless the toxin production in these 

 forms is in some way related to the creation of suitable metabolic 

 conditions for the organism itself, its biological purpose is quite 

 obscure. It is not impossible that by the destruction of hving 

 tissues these toxins create the conditions that permit development. 



4. Bacteria which ordinarily lead a purely saprophytic existence 

 but which, given suitable circumstances, may invade the tissues 

 of another form. This is the most widely distributed class of the 

 pathogenic organisms, and includes most of the ordinary intestinal 

 infectious agents and many of the so-called surgical infections; 

 and the bacteria of this class, because of their saprophytic attri- 

 butes, are easily cultivated on artificial media. 



5. The relatively strict parasites which cannot be cultivated at 

 all, or only by the use of specially adapted methods, and which 

 seem to have developed a more or less strict parasitism for the 

 conditions prevailing in the living bodies, often of a particular 

 animal species. 



Any classification of this kind must be regarded as repre- 

 senting points on a curve along which many gradations 

 of saprophytism and parasitism are recorded. In regard to 

 speed and delicacy of biological adaptation, there is no class of 

 living things more worthy of study than the bacteria. 



