266 BACTERIA 



the disease as cause to effect. In order to secure a standard 

 by which all investigators should test their results, Koch in- 

 troduced four postulates. Until each of the four has been 

 fulfilled, the final conclusion respecting the causal agent 

 must be considered sub judice. The postulates are as 

 follows : 



(a) The organism must be demonstrated in the circulation 

 or tissues of the diseased animal. 



(b) The organism thus demonstrated must be cultivated 

 in artificial media outside the body, and successive genera- 

 tions of &pure culture of that organism must be obtained. 



(c) Such pure cultures must, when introduced into a 

 healthy and susceptible animal, produce the specific disease. 



(d) The organism must be found and isolated from the 

 circulation or tissues of the inoculated animal. 



It is evident that there are some diseases for example, 

 cholera, leprosy, and typhoid which are not communicable 

 to lower animals, and therefore their virus cannot be made 

 to fulfil postulate (c). In such cases there is no choice. 

 They cannot be classified along with tubercle and anthrax. 

 Bacteriologists have little doubt that Hansen's bacillus of 

 leprosy is the cause of that disease, yet it has not fulfilled 

 postulates (&) and (c). Nor has the generally accepted 

 bacillus of typhoid fulfilled postulate (c), yet by the major- 

 ity it is provisionally accepted as the agent in producing 

 typhoid. Hence it will be seen that, though there is an 

 academical classification of causal pathogenic bacteria ac- 

 cording as they respond to Koch's postulates, yet neverthe- 

 less, there are a number of pathogenic bacteria which are 

 looked upon as causes of disease provisionally. Anthrax 

 and tubercle, with perhaps the organisms of suppuration, 

 tetanus, plague, and actinomycosis, stand in the first order 

 of pathogenic germs. Then comes a group awaiting 

 further confirmation. It includes the organisms related to 

 typhoid, cholera, malaria, leprosy, diarrhoea, and pneumonia. 



