2 Historical 



living organism but a noncrystalline chemical substance similar to 

 a precipitate of alumina. The chemists looked with contempt upon 

 the experiments of the biologists as frivolous and nonscientific. Fer- 

 mentation was considered a mysterious catalytic force. 



The plant physiologists, as typified by Dumas and Boussingault, 

 also failed to recognize the role of microbes in the cycle of life in 

 nature. They considered the animals and the plants the only living 

 forms that participate in this cycle. They stated, "Tout ce que I'air 

 donne aux plantes, les plantes le cedent aux animaux, les animaux 

 le rendent a I'air; cercle eternel dans lequel la vie s'agite et se mani- 

 feste, mais ou la matiere ne fait que changer de place" (Chemical 

 Statics of Organized Bodies). Matter was thus believed to be in 

 a state of change between plant and animal bodies. 



It was Pasteur, the microbiologist, who emphasized the micro- 

 biological nature of the processes of transformation of organic mat- 

 ter in nature. He established beyond question that, in addition to 

 plants and animals, a third group of living forms, the microbes, par- 

 ticipate in the cycle of life; they bring about the mineralization of 

 residues of both plant and animal life and retransform the elements 

 into forms available for fresh plant growth. 



Since the epoch-making contributions to the new science of micro- 

 biology by Pasteur and others, there have gradually emerged two 

 distinct phases of this science, the medical and the biochemical. 

 These have frequently overlapped and just as frequently diverged 

 from one another. Their attitudes toward the role of microorganisms 

 in natural processes can be summarized from their respective view- 

 points. 



1. Microorganisms as Disease-Producing Agents. The role of 

 microorganisms as causative agents of disease gave rise to a branch 

 of microbiology which is usually described by the terms "medical 

 bacteriology," "medical mycology," and "plant pathology." It deals 

 with the causation of disease, with infections and epidemics, among 

 human beings and other animals, as well as among plants. This 

 newly acquired knowledge has revolutionized our whole concept of 

 public health and disease. 



2. Microorganisms as Chemical Reagents. A study of the chem- 

 ical activities of microorganisms led to the development of microbial 

 physiology and biochemistry. It has resulted in numerous applica- 

 tions of microbiology to various fields of human endeavor, as indi- 

 cated by the terms "dairy bacteriology," "soil microbiology," "sewage 

 bacteriology," "microbiology of foodstuffs," and "industrial micro- 



