BACTERIAL METABOLISM WITHIX THE BODY 667 



in the presence of opposition from the various bactericidal forces of the 

 host, hut. they lack the power of independent iiivasiveness. They are "op- 

 portunists" with respect to pathogenicity and they are usually secondary 

 invaders hecause they require some break in the continuity of the skin or 

 mucous membranes to permit of their entrance to underlying tissues. Such 

 an organism is the Streptococcus. Parasitic bacteria do not ordinarily 

 incite epidemics, because they have not perfected a mechanism for escape 

 from the tissues, and as a general rule their excursion into the tissues 

 results in relatively non-specific inflammatory processes, rather than well- 

 defined anatomical lesions. 6 Recovery from an invasion of organisms of 

 the opportunist type does not ordinarily appear to result in a well-defined 

 specific immunity, thus again affording a contrast to bacteria of the pro- 

 gressively pathogenic type. 



Finally, the members of a small but formidable group of bacteria are 

 progressively pathogenic, that is to say, they appear to possess the power 

 of independent invasiveness of the body, if they reach a suitable portal of 

 entry in sufficient numbers. After invasion they multiply for a period of 

 time within the tissues of the body in the presence of the opposition offered 

 by the various non-specific lines of defense. They have individually per- 

 fected, finally, well-defined mechanisms of escape from the tissues to 

 channels in communication with the outside world, thus providing for 

 escape to other, susceptible hosts, and the perpetuation of the species. 



The typhoid bacillus may be cited as illustrative: The organism, must 

 reach the small intestine of a susceptible individual, penetrate the mucosa, 

 and enter the circulation. It grows in the tissues and, after a period of 

 time, reenters the intestines from the gall bladder from which it escapes 

 to the environment in increased numbers, or it escapes from the iirinary 

 bladder to the outside world. 



Thus, it is possible to distinguish a "cycle of parasitism" and a "cycle 

 of pathogenism." The essential factors of the former are first, for the 

 parasitic microbe to reach the surface of a suitable host, or to reach chan- 

 nels or cavities in free communication with the outside world; secondly, 

 for the microbe to multiply there, and, thirdly, to escape to other, suitable 

 hosts, thus insuring the perpetuation of the species. Penetration of the 

 tissues and growth therein is not a part of this cycle the microbe cannot 

 escape to the outside, as a general rule, and perishes, although it may 

 overwhelm the host in so doing. Parasitic organisms, therefore, are not 

 progressively pathogenic. The pathogenic cycle is somewhat more com- 

 plex. The organism must reach a suitable portal of entry to the under- 

 lying tissues of the host, actually penetrate into the underlying tissues and 

 grow therein in the face of non-specific and' specific opposition. Finally, 



Thus, the lesions caused by progressively pathogenic bacteria, as the tubercle, 

 typhoid, or syphilis microbes, are' fairly distinctive and characteristic in structure and 

 distribution, contrasting sharply in this respect with the non-specific inflammations in- 

 duced by streptococci or other pyogenic microbes. 



