106 INFECTION 



but several varieties of these bacteria invade the tissues and cause 

 infection. When one pathogenic microorganism, such as the typhoid 

 bacillus, has caused the primary infection, because of the local and gen- 

 eral conditions of lowered vitality of the tissues, these otherwise sa- 

 prophytic bacilli tend to intensify the infection. Blood infections, on 

 the other hand, are usually due to one form of bacteria, and even when 

 two or more varieties are introduced, only one, as a rule, is capable of 

 surviving and developing. The products of certain bacteria, on the 

 other hand, may immunize the host against infection with other bacteria, 

 for, as shown by Pasteur, attenuated chicken-cholera cultures may 

 produce immunity against anthrax. In the intestine harmless varieties 

 of bacteria may be made to crowd out more dangerous ones; this is 

 exemplified by the ingestion of soured milk which contains lactic-acid 

 bacteria, as advocated by Metchnikoff. 



SUMMARY 



From what has been said it is clear that infection differs from mere 

 surface contamination, and cannot be said to occur until the invading 

 bacteria have reached the deeper tissues, or a point where they may grow 

 and multiply. The surface epithelium and various secretions offer the 

 most potent local obstacles to infection, but even when these barriers 

 are broken down, the invaders may not survive the onslaughts of various 

 protective agencies of the host. In order to withstand and overcome 

 these attacks, the bacterium may undergo certain morphologic and 

 physiologic changes, and actively secrete a substance that is inimical 

 to the defensive forces of the host, or immunize itself against these forces. 

 Thus a certain species of bacteria may become selectively fortified or 

 immunized against a certain host or organ of that host, and show a 

 specific affinity for producing infection of a certain animal or a particular 

 organ. When the bacterium has overcome the defensive forces of a host, 

 it may, by the formation and. action of exogenous and endogenous toxins, 

 bacterial proteins, mechanical blocking of vessels, or formation of pto- 

 mains, produce disease. These various factors will be considered in 

 greater detail in the following chapter. 





