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pronounced. Much more spectacular is the result of a larger exper- 

 iment in which flasks with the same medium are inoculated with pure 

 cultures of Saccharomyces cerevisiae (baker's yeast), Lactobacillus delbriickii 

 (the so-called 'domesticated' lactic acid bacterium), Lactobacillus fer- 

 mentum (a 'wild' lactic acid bacterium), Bacterium coli, Bacterium aerogenes, 

 Bacterium typhosum, Granulobacter saccharobutyricum, and Granulobacter buty- 

 licum, and incubated under exclusion of air. After some time it will 

 be evident that the yeast has converted the sugar largely into ethanol 

 and carbon dioxide ; L. delbriickii into lactic acid ; L.fermentum into lactic 

 and acetic acids, ethanol and carbon dioxide; B. coli into lactic, acetic, 

 and succinic acids, carbon dioxide, and hydrogen; B. aerogenes into 

 the same products with, in addition, 2,3-butylene glycol; B. typhosum 

 into formic, acetic, and lactic acids, and ethanol; G. saccharobutyricum 

 into butyric and acetic acids, carbon dioxide, and hydrogen; G. buty- 

 licum into butanol, acetone, carbon dioxide, and hydrogen. Thus a 

 remarkable diversity emerges. 



Nevertheless, this diversity in products obtainable from a single 

 substrate is almost negligible in comparison with the differences that 

 various microbes exhibit with respect to their nutrient requirements. 



For their studies on micro-organisms the early microbiologists de- 

 pended more or less on the fortuitous appearance of special types. But 

 gradually they became aware of correlations between the initial com- 

 position of the medium and the microbes therein encountered. To 

 mention just one example: it soon became clear that true yeasts are 

 found only in sugar media. After the introduction of pure culture 

 methods had permitted a closer investigation of metabolic activities, 

 it became increasingly evident that substances which represent excel- 

 lent foodstuffs for one type of microbe may be less so, if not entirely 

 unsuitable, for others. Once this had been recognized, the idea ob- 

 viously occurred that the investigator possessed a powerful tool for 

 encouraging the development of certain microbes at the expense of 

 others present in the inoculum. Although ultimately this method of 

 elective cultures harks back to Pasteur and Raulin, it is in the hands 

 of Beijerinck and Winogradsky that it has indubitably produced the 

 richest harvest. Well-nigh overwhelming is the metabolic diversity 

 revealed by the consistent application of their 'enrichment cultures' ; 

 some examples may illustrate this. 



Based on Berthelot's studies, showing that the increase in bound 



