Depending upon the ability to grow in the body, bacteria may 

 be divided into: (i) purely saprophytic ; (2) occasionally para- 

 sitic; <ind (3) purely parasitic. A host harbors a parasite. 



Purely saprophytic germs cannot live in tissues at all; those 

 that are occasionally parasitic lead a saprophytic existence in 

 the soil or water, and yet may invade the body, and produce 

 disease: the tetanus and malignant oedema bacilli are examples 

 of this group. Those bacteria that are purely parasitic are only 

 known as they exist in the tissues of the infected host, and have 

 no outside existence at all. 



Koch's Postulates 



In order to prove that a certain organism is the infectious agent 

 of a given disease, Koch has devised four postulates which the 

 given organism must fulfill before it can be considered the cause 

 of the disease. 



1. The organism must be found microscopically in the tissues 

 of the animal having the disease, and its position in the lesion 

 should explain the latter. 



2. It must be isolated in pure state from bodies of the diseased 

 animals. 



3. And then it must be grown for successive generations in 

 culture media. 



4. If injected into a healthy animal, or animals, it must produce 

 the same disease, and be found in the lesions of the disease in 

 the animal's tissues. 



Some of the many organisms that certainly fulfill these condi- 

 tions, are as follows : 



Streptococcus Pyogenes (Sepsis). Actinomyces. 



B. of Tuberculosis. B. of Diphtheria. 



B. of Anthrax. B. of Tetanus. 



B. of Glanders. B. of Malignant (Edema 



B. of Bubonic Plague. B. of Malta Fever. 



