90 SAPROPHYTISM, PARASITISM, AND PATHOGENISM 



man are found in Nature far removed from rather intimate association 

 with their hosts. This is in accordance with the fact that few, if any, 

 of these organisms are provided with spores which would enable them 

 to survive exposure to long periods of conditions unfavorable to their 

 growth. It is true, however, that some, at least, of these organisms, 

 as for example, the typhoid bacillus, can survive for longer or shorter 

 periods of time in the soil, particularly if it be frozen, or in water, for 

 days or even weeks. There is little evidence that these bacteria multiply 

 extensively outside the body; on the contrary, they tend to die off 

 rather rapidly. In any event, their existence depends upon their 

 reaching a suitable host again within a comparatively brief period. 



There are a few spore-forming bacteria which occasionally infect 

 man when associated conditions are favorable for them. Of these 

 the bacillus of lockjaw, B. tetani; of botulism, B. botulinus; the 

 gas bacillus, B. aerogenes capsulatus; and the anthrax bacillus are 

 well-known. These organisms are not habitual parasites, however; 

 they are "saprophytic opportunists." That is, they could in all 

 probability exist if man were eliminated from their environment. 



V. HOW PARASITIC AND PATHOGENIC BACTERIA REACH MAN. 



A. The Occurrence of Parasitic Bacteria upon the Bodies of Healthy 

 Men and Animals. The continual exposure of the skin of man to his 

 environment makes it almost inevitable that microbes shall collect 

 there. It is quite probable, however, that the large number of micro- 

 organisms which reach the skin are not only non-pathogenic, they are not 

 even habitually parasitic. Most of them are found there only trans- 

 iently. Certain organisms, however, occur among these adventitious 

 microbes, which appear to be habitual parasites, and many of these 

 bacteria, under certain conditions, produce disease. Of these, Staphy- 

 lococcus aureus and albus and Streptococcus pyogenes are almost 

 invariably present not only on the skin, but on the exposed mucous 

 membranes, particularly those of the nose and throat. The influenza 

 bacillus, diphtheria bacillus, the pneumococcus, and even the tubercle 

 bacillus, meningococcus and other organisms may also be occasionally 

 found, particularly in the nose and throat of healthy men. The 

 occurrence of these organisms is readily explained; the secretions of 

 the nose and throat, as well as that of the skin are excellent culture 

 media for these organisms, which collect at these sites and grow upon 

 the various secretions and desquamated cells. 



