MECHANISM OF BACTERIAL INVASION 91 



cells offer a mechanical obstacle to invasion. This resistance is naturally 

 more complete where the cells are thickened and most compact. In the 

 depths of glands and in mucous membranes, where numerous glands are 

 present, and where the layers are thinner and moisture exists, the 

 barrier is less complete. 



2. Surface discharges are potent factors in preventing bacterial 

 invasions by (1) Washing away the bacteria mechanically; (2) by 

 germicidal activity through the presence of various chemical agents, 

 such as acids, which they may contain, and (3) by antiseptic and 

 even bactericidal substances that may be present in the form of anti- 

 bodies. 



The saliva, with its antiseptic and germicidal properties, is potent 

 in preventing infections of the mouth and upper air-passages; when 

 this secretion is diminished, as during the course of high fever, bacterial 

 activity is enhanced, which is evidenced by the development of fetid 

 sordes about the teeth and on the lips. 



The acidity of the gastric juice and its germicidal powers are well 

 known and appreciated; similarly the urine, the milk, and to a slight 

 extent, the bile, have been demonstrated by 'Adami to exert a distinct 

 antiseptic effect upon certain bacteria, such as the Bacillus coli. 



Surface moisture and discharges about the nose and throat are also 

 potent factors in mechanically removing bacteria from inspired air, and 

 no doubt frequently prevent bacterial invasion of the lower respiratory 

 tract, where more mischief may be done. 



3. The cells of certain excreting glands may possess bactericidal and 

 excretory powers of value in preventing bacterial invasion (Adami). 



MECHANISM OF BACTERIAL INVASION 



We will now consider the method by which invasion, the first step 

 of what may be an infection, is brought about. In brief, one or all of 

 the normal defenses just described must be overcome; in some instances 

 the microorganisms, by their inherent disease-producing powers, may 

 accomplish this unaided; in other instances the resistance is overcome 

 by a general lowering of the vitality of the body defenses. 



1. Traumatic solution of the surface layers of epithelial cells is a 

 very important factor in the production of infection, as the invading 

 microparasites are thus given easier access to the deeper and less resistant 

 tissues. The pathologist or surgeon may, in the course of his work, 

 contaminate his hands with secretions containing virulent microorgan- 



