62 Perspectives in Microbiology 



which even superficial examination shows that much re- 

 mains to be done. 



A glance at the history of microbiology shows that the 

 development of knowledge of bacterial metabolism has 

 passed through several more or less distinct but overlapping 

 phases, characterized by differences in the objectives, the 

 points of view, and particularly the experimental methods 

 of the investigators. 



The first phase can be described as that of exploration. 

 Led by such pioneers as Pasteur, Winogradsky, and Bei- 

 jerinck, studies in bacterial metabolism were concerned 

 with the role of bacteria in a great variety of naturally oc- 

 curring chemical processes like the souring of milk, the 

 formation of nitrate in the soil, and the accumulation of 

 hydrogen sulfide in Dutch canals. During this period many 

 bacteria were discovered, and most of the basic and now 

 familiar facts of bacterial physiology were established. The 

 more obvious nutritional requirements of bacteria were 

 determined; the existence of aerobic and anaerobic, auto- 

 trophic and heterotrophic, chemosynthetic and photosyn- 

 thetic types of metabolism was demonstrated. Most of the 

 presently known physiological groups of bacteria were 

 recognized, and their main substrates, catabolic products, 

 and environmental requirements were established at least 

 qualitatively. 



The exploratory phase of microbiology, of course, did 

 not come to a sudden close, but has continued with decreas- 

 ing activity to the present time. Within the last twenty-five 

 years, numerous notable contributions have been made. As 

 illustrations, I shall mention only the brilliant studies of 

 van Niel and his associates on the photosynthetic bacteria, 

 and of Hungate (11) and of Sijpesteijn (22), independently, 

 on the cellulose-fermenting bacteria of the bovine rumen. 

 These and other studies have shown that the exploratory 

 phase of microbiology is by no means past, although it no 

 longer occupies the center of interest. 



