48 OCCURRENCE OF PATHOGENIC BACTERIA IN NATURE 



may spread the disease wherever the hay is taken. Milk from tuber- 

 cular cows may contain the tubercle bacillus, and when used in the 

 raw state may spread the disease to animals, particularly to young 

 calves and hogs, and man. Pathogenic bacteria which are excreted 

 with the feces, urine, and other discharges may contaminate the floors 

 and walls of houses, barns, stables, and also the bedding, straw, hay, 

 and manure, and may exist on or in these objects for a long time. 

 In cadavers properly buried pathogenic bacteria do not remain very 

 long, not over a few months at most. 



Portals of Entrance for Pathogenic Bacteria. Pathogenic bacteria, 

 whether they exist in nature as saprophytes like the tetanus or anthrax 

 bacilli, or are there temporarily like the tubercle and glanders bacilli, 

 or are directly disseminated from a sick to a healthy living being, 

 must always enter a susceptible animal through certain portals of 

 entrance in order to find the proper conditions for invasion and 

 subsequent growth and multiplication. Sometimes a pathogenic 

 bacterium may gain entrance by several routes, and sometimes it 

 can enter but by one. For example, as far as known, the tetanus 

 bacillus never infects a horse through the respiratory tract by inha- 

 lation nor through the gastro-intestinal tract by ingestion. It must 

 always, in order to produce its disease, enter through a wound. 

 Likewise, the typhoid bacillus and the spirillum of Asiatic cholera 

 can infect man only through the gastro-intestinal tract by ingestion. 

 On the other hand, the anthrax bacillus may invade the tissues 

 through any one of these three channels. 



Skin. The intact skin with its outer layers of cornified epithelial 

 cells forms an almost perfect barrier against the invasion of patho- 

 genic bacteria. However, slight breaks in the skin are quite common, 

 and these form the portals of entrance for a host of wound infections 

 by the pyogenic cocci, the tetanus, anthrax, malignant edema, 

 black-leg, necrophorus bacilli, by the organism of botryomycosis. 

 Sometimes an animal may even be infected through the intact skin, 

 particularly if the pathogenic organisms are rubbed in. In this way 

 guinea-pigs may be infected with the bacillus of bubonic plague. 

 While open, and particularly small, deep-seated puncture wounds 

 offer a very favorable means of entrance to many pathogenic bac- 

 teria; it has been found that a wound once covered with granulation 

 tissue becomes impenetrable. Evidently the granulations with their 

 numerous young connective tissue, vascular endothelial cells and 

 polyniiclear leukocytes form a barrier which cannot be broken 

 through. 



Mucous Membranes. The mucous membranes of the body are 

 much less resistant than the skin. Some pathogenic bacteria can 

 invade certain animals even through an intact mucous membrane. 

 For example, if glanders bacilli are placed on the conjunctiva of a 

 guinea-pig this animal contracts glanders; if plague bacilli are placed 

 on the conjunctiva of a rat, the latter will contract bubonic plague. 



