98 BACTERIOLOGY FOR NURSES 



accounts most readily for the far greater frequency with which 

 tuberculosis affects the lungs than it does other parts of the body. 

 Recent experiments, however, tend to show that they may be 

 swallowed and pass through a practically intact intestinal wall 

 and find their way into the mesenteric lymph glands. The cer- 

 vical glands of children often become infected with tubercle 

 bacilli without any visible trace being left in the mucous membrane 

 of the larynx to indicate their passage. In such cases the crypts 

 of the tonsils are strongly suspected of being their portal of en- 

 trance. 



Bones are infected by organisms carried to them in the blood 

 stream except in cases of direct injury ; for this reason the perios- 

 teum and the bone marrow, being most abundantly supplied with 

 blood, are first affected. 



Influence of Numbers. The number of pathogenic bacteria 

 which succeed in invading the tissues is an important factor in 

 determining whether or not an infection will take place. If only 

 a few gain entrance all may be killed ; if, however, a large number 

 are introduced some are almost sure to survive, to multiply, and 

 produce their specific disease unless the body is immune. When a 

 chronic disease or other factors have reduced the body defenses 

 fewer bacteria than would otherwise be required may give rise to 

 infection. 



Virulence. The degree of ability which an organism possesses 

 to overcome the defensive forces of the body and to give rise to 

 disease is spoken of as its virulence. Bacteria whose virulence 

 is great may produce disease when only few in number, whereas 

 millions of a less virulent species might be required. 



It is impossible, by any means, to make a known non-virulent 

 organism virulent. It is not difficult, on the other hand, to decrease 

 or increase the virulence of an organism already possessing patho- 

 genic powers. The ability to produce poison may be lessened 

 by repeated growth on artificial culture media ; by exposure of a 

 culture for a short period to a temperature just below the thermal 

 death point, or to sunlight, or to small quantities of antiseptic 

 or germicidal substances. These methods are frequently employed 



