THE PROBLEM OF VIRULENCE 25 



testinal bacteria may often penetrate into the portal circulation is 

 indicated by the occasional occurrence of colon bacillus abscesses 

 after trauma of the liver. In most septicemias, however, caused by 

 virulent bacteria the invasion of the blood stream persists, rapid 

 multiplication occurs and leads to death. 



From the circulation the bacteria may gain lodgment in various 

 organs and cause the formation of secondary abscesses. This condi- 

 tion is known as "pyemia," and may be caused by almost any bac- 

 teria which are capable of producing septicemia. Thus staphylo- 

 cocci, streptococci, or pneumococci may lodge in bones, joints, brain, 

 or kidneys, in fact in any organ in which they can gain a foothold. 

 However, there are evidences of distinct tissue predilections on the 

 part of certain germs. Thus the virus of rabies and that of polio- 

 myelitis, though to some extent universally distributed, seem espe- 

 cially to concentrate in the nervous system; cholera spirilla and 

 dysentery bacilli appear to find conditions most favorable for de- 

 velopment in the intestinal mucosa; amebic abscesses are most 

 common in the liver; gonococcus infections when generalized find 

 secondary localization with particular frequency on heart valves and 

 joints ; leprosy bacilli have a predilection for the nerve sheaths ; and 

 glanders bacilli injected into the peritoneum of a male guinea pig 

 localize with such regularity in the testicles that the experiment has 

 diagnostic value (Strauss test). Conversely it is only explicable on 

 the assumption of such selective lodgment that tubercle bacilli, even 

 though otherwise universally distributed through the body, will be 

 absent from striped muscle tissue, and rare in the walls of the stom- 

 ach. Such selection as far as we can account for it at all, seems to 

 depend upon the varying cultural conditions encountered by the 

 germs in different organs. 



On the other hand, localization may also be dependent upon 

 accidental conditions such as trauma. Infections in which the en- 

 trance of bacteria is coincident with injury as in the case, for in- 

 stance, of compound fractures will be able to spread throughout 

 the injured region much more easily than they could enter the 

 healthy tissue. In fact, it is well known that local tissue injury at 

 the point of inoculation favors infection since it furnishes a rich 

 substratum for growth in the form of dead cells or blood clot and 

 interferes with the accomplishment of a normal protective reaction. 

 In cases in which bacteria are circulating in the blood mechanical 

 injury may create a focus of reduced resistance on which the in- 

 vaders can gain a foothold. It is in this way perhaps that, among 

 other things, we can explain tuberculosis of joints or bones which 

 present a history of injury preceding the development of the infec- 

 tion or the pleurisy and lobular pneumonias which have been known 

 to ensue upon the fracture of a rib. 



It is also possible that bacteria may be distributed in various 



