362 METABOLISM OF MICROORGANISMS 



some microorganisms work so fast is that they derive very little energy 

 from the chemical changes that they bring about. Hence, of necessity, 

 they must work over a large amount of matter in order to meet their 

 energy needs. 



Comparison of microorganisms with respect to end products 



The term fermentation may be defined as the chemical process by 

 which organic compounds are converted into new compounds by micro- 

 organisms or by enzymes obtained from microorganisms. With modi- 

 fying words it is used in a broad sense to designate products formed, 

 materials utilized, or agents involved. When dealing with products 

 formed, we have such phrases as alcohol fermentation, citric acid fer- 

 mentation, penicillin fermentation, tea fermentation, etc. To feature 

 the materials utilized, such terms as glucose-, xylose-, cellulose-fermenta- 

 tion are used. In designating the microbiological agent such expressions 

 as yeast fermentation, bacterial fermentation, and mold fermentation are 

 employed. In this chapter the word fermentation is used in all three 

 of these ways. No distinction is made among processes that are 

 anaerobic and produce gas, e.g., alcoholic fermentation, those that are 

 anaerobic and produce no gas, e.g., lactic acid fermentation, and those 

 that are aerobic and produce gas (carbon dioxide), e.g., citric acid and 

 penicillin fermentations. 



In a restricted sense the term fermentation is used to denote an 

 anaerobic type of metabolism. Associated with this usage is the term 

 respiration, brought over from animal metabolism to denote what in 

 effect is complete oxidation of the substrate to carbon dioxide and water. 

 Attempts to separate microbial metabolism into fermentation and respira- 

 tion processes seem highly artificial, since the yeast cell, for example, 

 may operate on either an aerobic system or an anaerobic system, and at 

 times even on both systems simultaneously. 



In Table 14-1 are listed the products that are characteristic of bacteria, 

 yeasts, and molds. The percentage given for the glucose or fructose 

 converted into the corresponding product is maximal, or nearly so. It 

 is believed that figures showing such a performance of a microorganism 

 are more meaningful than data obtained under conditions that do not 

 permit the cells to function at or near their optimal capacity. 



An inspection of this table shows that there are about a half dozen 

 products that are common to all three groups. Carbon dioxide is the 

 compound that is not only common to all three, but is also produced 

 in large amounts by many members of each group. It is probably a 

 universal product of cell metabolism. In certain cases a product is 

 more generally found in one or two of the groups rather than in the 



