CHAPTER V. 



OCCURRENCE OF PATHOGENIC BACTERIA IN NATURE- 

 ROUTES OF ENTRANCE IN INFECTION. 



MANY pathogenic bacteria are only facultative, and not strict 

 parasites. They are not only found in infected persons and animals, 

 but also in the outside world, where as saprophytes they find 

 all conditions necessary for their existence, growth, and multipli- 

 cation. There are a number of bacteria of this type which have an 

 extensive distribution in nature and only occasionally invade the 

 organism of animals, to lead there a parasitic existence. Among these 

 we may mention the pus-producing (pyogenic) staphylococci. They 

 are truly ubiquitous, and are found in the air, soil, water, on the external 

 surfaces of all animals, in the various parts of their gastro-intestinal 

 and respiratory tracts, and on the outside surfaces of all objects in 

 nature, whether animate or inanimate. 



Such organisms as the tetanus bacillus, the bacillus of malignant 

 edema, and the Bacillus enteritidis sporogenes are prevalent in the 

 soil; the first one of the three also occurs extensively in the intestines 

 of the horse as a harmless commensale. The ray fungus is found in 

 nature on many grasses, and the anthrax bacillus on pastures, where 

 it multiplies and leads a saprophytic existence. 



Certain pathogenic bacteria are only found in the outside world in 

 the neighborhood of infected persons and animals which are respon- 

 sible for their dissemination. In this manner typhoid bacilli are 

 sometimes transferred to the soil, and the spirilla of Asiatic cholera 

 to stagnant waters or even rivers, notwithstanding the fact that 

 rapidly flowing rivers generally rid themselves quickly of pathogenic 

 bacteria through the effect of sunlight and by the aid of algae which 

 destroy them. The anthrax bacillus, while undoubtedly found in 

 nature independent of anthrax-sick animals, is often much dissem- 

 inated by the latter. 



There are, on the other hand, certain strictly parasitic pathogenic 

 bacteria only found in the neighborhood of sick persons or animals 

 from which they may spread for a short distance. Tubercle bacilli 

 and glanders bacilli belong to this type and cannot, as far as known, 

 exist in the outside world. Their presence in it is directly or indi- 

 rectly dependent upon beings suffering with their specific diseases. 



Pathogenic bacteria in the outside world may exist in or on the 

 food of man and animals. The hay harvested from anthrax-infected 

 prairies and pastures will contain this bacillus and its spores, and 



