38 Microbes and You 



Koch emphasized the importance of proving concepts by employ- 

 ing the scientific approach of actual laboratory experimentation. 

 Before an organism could be said to be the cause of a specific 

 disease, the agent had to fulfill the following postulates which Koch 

 formulated in 1882: 



1. The suspected organism must be found in every case of the 

 disease. 



2. The organism must be isolated in pure culture from every case 

 of the disease. 



3. These isolated pure cultures, when introduced into susceptible 

 animals, must be capable of reproducing the original disease 

 in its typical clinical form. 



4. The same organism must be re-isolated from the injected test 

 animal. 



If the postulates were fulfilled, Koch was willing to admit that 

 the cause of that particular disease had been proven. We should 

 point out here, however, that Koch's postulates cannot be applied 

 in some cases. When you try to select a susceptible animal in 

 postulate three, you find that human beings are the only animals 

 you can use in some diseases and that is not always practical. It 

 can be said, however, that accidental laboratory infections have 

 permitted proof of the etiology of some diseases, such as t)^phoid 

 fever, which man alone seems to contract. The strictly parasitic 

 nature of such microorganisms as the leprosy organisms does not 

 allow their isolation on ordinary laboratory media, and that sets 

 up a block in the necessary cycle of proof. There are some 

 diseases in which the clinical symptoms are the result of multiple 

 infections, and trving to prove that a single organism is the cause 

 of the disease may lead to confusing results. The common cold, 

 for example, probably is initiated by a virus, or bv viruses, but the 

 misery of a cold seems to be associated with the activitv of 

 secondary invaders, usually bacteria, which are opportunists. 



In 1863 Casimer Joseph Davaine ( 1812-1882 ) reported that the 

 blood of animals infected with anthrax contained rod-shaped 

 organisms which could be transferred bv blood from infected 



