8 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



a physician by the name of Billroth concluded that all bac- 

 teria were different growth forms of one single pleomorphic 

 species, which, when growing under different conditions 

 shows different cell forms. His suggestion was made be- 

 fore modern laboratory technic was well developed. Much 

 of the literature, particularly the medical literature, in the 

 two decades following the work of Billroth is greatly con- 

 fused because of adherence to this doctrine Here again 

 laboratory studies carried out in the most careful manner 

 were necessary to determine the right. They led to the 

 discovery of many new facts regarding microorganisms, and 

 showed quite conclusively that there are many distinct 

 species of bacteria, and that Billroth was mistaken in his 

 assumption that all were growth forms of a single pleo- 

 morphic species. It is true that microorganisms sometimes 

 vary in their shape, their appearance, and their size, when 

 grown under different conditions, and sometimes they can 

 be distinguished from each other only with difficulty. 

 Nevertheless there are many distinct kinds, and the 

 theory of pleomorphism in its original form has been dis- 

 proved. 



The Controversy over the Germ Theory of Disease.— 

 The fact that bacteria can produce disease was first ade- 

 quately demonstrated and proved by Robert Koch in 1872. 

 Before this time there had been occasional conjectures that 

 microorganisms might have something to do with disease 

 but no real proof. Koch examined microscopically the 

 blood of cattle having the disease called anthrax, and ob- 

 served numerous rod-shaped organisms among the red blood 

 cells. These he succeeded in transferring to culture media 

 and in growing them free from every other kind of organ- 

 ism, that is, in pure culture. He found that he could culti- 

 vate them in this fashion indefinitely in the laboratory, and 

 that whenever they were introduced under the skin of sheep 

 or cattle the animal would invariably contract the disease 



