18 INFECTION AND RESISTANCE 



In order that a micro-organism may be a true parasite in Bail's 

 sense or invasive for any given species of animal it must of 

 course possess certain basic cultural attributes which enable it to 

 grow in the environment furnished by the host. For instance, a 

 micro-organism which does not grow at temperatures below 37.5 C. 

 cannot very well become parasitic upon cold-blooded animals. An 

 excellent illustration of this influence of body temperature upon 

 the invasive powers of bacteria is furnished by the different races 

 of acid-fast bacilli which invade the bodies of man and of birds. 

 The avian tubercle bacillus, for instance, is non-pathogenic for man 

 and in cultures will not develop at temperatures below 40 C., 

 which is about the body temperature of most birds. The human 

 tubercle bacillus, on the other hand, is non-pathogenic for birds and 

 ceases to grow in artificial cultures when the temperature is raised 

 above 40 to 41 C. This is merely one of a number of examples 

 which might be cited to demonstrate the necessity of simple cultural 

 adaptation, as it influences the property of virulence. Again, it is 

 probable that in order to develop in the animal body it is necessary 

 that a micro-organism shall be capable of developing without free 

 oxygen. While this point is not definitely certain, it is not probable 

 that any of the virulent bacteria can be strict aerobes. As a matter 

 of experience none of the pathogenic bacteria at present known are 

 absolute aerobes though many of them grow better in artificial cul- 

 ture when oxygen is freely present than when it is absent. 



Furthermore, the conditions encountered by bacteria as they 

 enter the animal body will vary considerably according to the path 

 by which they gain entrance. Organisms entering by the intestinal 

 canal are subjected to conditions of acidity or alkalinity, the action 

 of digestive juices, of bile, and to competition with other intestinal 

 bacteria, forces to which many pathogenic germs will succumb, while 

 others may survive there and thrive. Those entering into the tissues 

 by way of the skin and mucous membrane, on the other hand, en- 

 counter an immediately mobilized protective mechanism which, suc- 

 cessfully resisted by some of them, might easily and quickly dispose 

 of small quantities of other bacteria more resistant to conditions in 

 the bowel. It is but natural for this reason that the accomplishment 

 of an infection by any given germ must depend to a great extent 

 upon its gaining entrance to the body by the path best adapted to its 

 peculiar requirements. 



The mechanical protection afforded by the coverings of skin and 

 mucous membranes is as a rule sufficient to prevent the penetration 

 of any bacteria which by chance may have found lodgment upon 

 them. In the case of the most usual pyogenic cocci and many bacilli 

 such protection is probably absolute, and a distinct break of con- 

 tinuity, such as a bruise or a wound, even though this may be too 

 small to attract attention, is necessary for successful infection. In 



