JO PATHOGENIC BACTERIA. 



Ordinarily, however, number plays a very important 

 part in the process of infection. Park explains this by 

 calling to mind what happens and can easily be de- 

 monstrated in the transplantation of bacteria to fresh 

 culture-media. When such a transplantation is made 

 the greater number of the transplanted organisms 

 quickly die. Those that live grow rapidly, so that 

 while a short time after transplantation the number 

 of living bacteria is less than the number actually put 

 in, in a few hours these have increased amazingly. 

 "With those bacteria whose virulence is great — i. e. those 

 which are capable of growing with great ease in the body- 

 fluids — a very few organisms will produce disease almost 

 as quickly as a million, allowance only being made for 

 the short time required for a few to become equal in num- 

 ber to the million. At the other extreme of virulence, 

 however, many millions may have to be introduced to 

 permit of the development of any of the organisms in the 

 body." 



It can be shown experimentally that a certain number 

 of the pyogenic cocci can be injected into the perito- 

 neum of a rabbit without provoking peritonitis, but that 

 if this number be exceeded the animal will die. 



The effect of number must not be construed, however, 

 to mean that the greater the quantity of culture injected 

 the more rapid the outcome. After a fatal dose of any 

 culture has been given to an animal a certain length of 

 time must elapse for the development of pathogenesis, 

 and doubling or trebling the dose will scarcely hasten the 

 fatal outcome. This is well illustrated in the case of 

 guinea-pigs and mice, which, when injected with anthrax, 

 die in about twenty-four hours, whether the dose be ten 

 times or a million times that which is fatal. 



3. Avenue of Infection. — It makes a great difference in 

 the subsequent events whether the infection takes place 

 through usual or unusual channels. Thus, in the case 

 of tuberculous infection of man, when the bacilli are 

 inhaled or ingested and reach the internal organs, the 



