PARASITIC AND PATHOGENIC BACTERIA 97 



The girat majority of specific microbic diseases (in contradistinction 

 to non-specific inflammations) are incited by bacteria of exogenous 

 origin. These organisms must enter the host directly through their 

 respective appropriate atria to produce characteristic disease. For 

 example, the typhoid bacillus only causes typhoid fever when the 

 organism is swallowed and enters the body through the intestinal 

 tract. Infection of a skin- wound with typhoid bacilli will not result 

 in typhoid fever. Similarly, cholera vibrios do not produce the disease 

 cholera unless they enter the body through the gastro-intestinal tract, 

 although if cholera vibrios are introduced through the skin in experi- 

 mental animals they tend to migrate toward the intestinal tract, thus 

 suggesting a special affinity for the intestinal tissues. Pathogenic 

 bacteria of exogenous origin produce in general, progressive specific 

 disease from man to man. Bacteria of endogenous origin, on the 

 other hand those which occur habitually as " opportunists" on the 

 surface of the body or on mucous membranes opening to the exterior 

 ordinarily exist as harmless parasites. They may, however, and 

 occasionally do, become invasive, inciting local or generalized inflam- 

 matory reactions as a rule, rather than well-defined clinical syndromes 

 which are frequently so characteristic of infections with exogenous 

 pathogenic bacteria. The bacteria of the "opportunist" type do not 

 ordinarily gain entrance to the tissues of the body through sharply- 

 circumscribed atria and the disease they produce is usually not epidemic 

 in character. 



1. Skin and Adnexa The intact skin is a natural barrier which 

 protects the underlying tissues of the body from bacterial invasion. 

 Its free exposure to the environment suggests that a great variety of 

 organisms find lodgment upon it from time to time; a majority of 

 these organisms are harmless, and probably transient saprophytes 

 which come and go irregularly. The moisture and excretions, how- 

 ever, appear to favor the limited development of a few types of bac- 

 teria, mainly those of the coccal group, which occur with sufficient 

 regularity to be regarded provisionally as habitually parasitic bacteria. 

 Of these, the pyogenic cocci are usually the most numerous; they 

 exist as "opportunists" on the surface of the skin or penetrate into 

 hair follicles and the ducts of the cutaneous glands, ordinarily, however, 

 without becoming invasive so long as the continuity of the skin is 

 maintained. Abrasions and cuts furnish a portal of entry to the sub- 

 cutaneous tissues, in which these parasitic bacteria frequently set up 

 inflammatory reactions. Friction may actually force them through 



