96 BACTERIOLOGY FOR NURSES 



transmitted by the latter method may under certain conditions 

 be transmitted by the former. A simpler plan is the classification 

 of all bacterial diseases as infectious, reserving the term contagious 

 only for those which are contracted as a result of direct contact. 



Defensive Forces of the Body. The body possesses three 

 natural defenses against bacterial invasion : (1) a covering more or 

 less unsuitable for bacterial growth and penetration; (2) the 

 ability to produce chemical substances which either kill the organ- 

 isms or render their poisons inert ; (3) the power of certain cells to 

 engulf and destroy the invaders. Thus even though the first 

 barrier be passed invasion does not necessarily mean infection; 

 the other forces acting singly or together may speedily prevent 

 injury. 



Influence of Tissues on Bacterial Invasion. Many species of 

 bacteria find a temporary lodgment upon the skin, but it is a poor 

 soil for growth and forms an effective barrier against entrance 

 into the deeper tissues. A group of cocci, however, are habitually 

 present, and injuries such as wounds, or burns, or sometimes a 

 simple pin prick enable them to penetrate deeper. Certain varie- 

 ties are able to produce direct action without the existence of 

 previous injury. Thus staphylococci may reach the roots of hair 

 follicles and sweat glands and cause suppurative conditions. 



The warmth and moisture of the mucous membranes make 

 them much better adapted for bacterial growth than the skin. 

 The nasal cavity is somewhat cleansed by the nasal secretion. 

 Nevertheless, the influenza bacillus, the diphtheria bacillus, and 

 others find it possible to obtain a lodgment and by concentrating 

 at one point to lower the vitality, destroy the epithelial tissue, and 

 gain an entrance there. It is possible that the meningococcus 

 and the virus of anterior poliomyelitis gain access to the body by 

 this route. 



Diminution in quantity or change in quality of the normal 

 body secretions may favor the growth of bacteria and render in- 

 vasion easier. For example, the saliva is ordinarily somewhat 

 bactericidal, but during a fever the amount secreted is diminished 

 and unless the mouth is carefully and frequently cleansed fetid 



