THE RELATION OF BACTERIA TO DISEASE 143 



Let us now consider some of the facts which have been observed 

 concerning the growth of bacteria in the living body as contrasted with 

 culture media. In the first place, it has been learned, as will be described 

 in the latter part of the book, that each variety of bacteria can incite 

 only certain types of infection. Indeed, because of this fact, the majority 

 of bacteria which excite disease can be traced back for thousands of 

 years by means of the records, these parasitic bacteria breeding true 

 and keeping distinct from the great mass of bacteria occurring in the 

 air, water, and soil. 



Parasitic bacteria have gradually adapted themselves not only to 

 certain species of animals, but to certain circumscribed areas of the 

 body. Thus the diphtheria bacilli grow chiefly upon the mucous mem- 

 branes of the respiratory tract, but cannot develop in the blood or in 

 the subcutaneous tissues. The cholera spirilla develop in the inflamed 

 intestinal mucous membrane, but cannot grow in the respiratory tract, 

 blood, or tissues. The tetanus bacilli develop in wounds of the sub- 

 cutaneous tissues, but cannot grow on the intestinal mucous mem- 

 branes or in the blood. 



Other bacteria find, indeed, certain regions especially suitable for 

 their growth, but under conditions favorable for them are capable of 

 developing in other locations. Thus, the typhoid bacillus grows 

 most luxuriantly in the Peyer patches and mesenteric glands, but also 

 invades the blood, spleen, and other regions. The tubercle bacillus 

 often remains localized in the apex of a lung or a gland for years, but 

 at any time may invade many tissues of the body. The gonococcus 

 finds the mucous membrane of the genitourinary tract most suitable 

 for its development, but also frequently is capable of growth in the 

 peritoneum and even sometimes in the general circulation. The pneu- 

 mococcus develops most readily in the lungs, but also invades the con- 

 nective tissues, serous membranes, and the blood. 



All these bacteria, although ordinarily increasing only in the body of 

 man, can be grown on suitable dead material. 



There are bacteria which, in so far as we know, find the bodies of 

 human beings or animals the only fit soil for their growth. These are 

 strictly the true parasites. The spirillum of relapsing fever grows only 

 in man; neither the food nor the conditions suitable for the develop- 

 ment of this micro-organism outside of the body have as yet been dis- 

 covered. 



Following rather closely the schematic separation of bacteria accord- 

 ing to their relation to disease we might classify them as : 



1. Strict saprophytes, or bacteria which grow readily in suitable dead 

 organic material, but not in the body under ordinary conditions. 



a. Bacteria which in their growth produce no substances poisonous 

 to the body, which are capable of absorption through the intestinal 

 walls or act on its epithelium. 



b. Bacteria which produce in their growth in dead organic matter 

 poisons capable of acting on the mucous membrane or of being 

 absorbed into the animal body. 



