364 METABOLISM OF MICROORGANISMS 



AEROBIC METABOLISM OF CARBOHYDRATES 



The conventional and most convenient method of classifying the varied 

 types of metabolism performed by microorganisms is on the basis of 

 utilization of oxygen. If oxygen is used, the metabolism is called aerobic ; 

 and if not, it is designated as anaerobic. This is an arbitrary classifica- 

 tion, as many organisms have both an aerobic and an anaerobic system 

 and operate on one or the other as circumstances require. Examples of 

 such microorganisms are E. coli and ordinary baker's yeast, *S. cerevisiae. 

 Bacteria that can grow either in the presence or absence of air are termed 

 facultative aerobes or facultative anaerobes depending upon which con- 

 dition appears more favorable. 



By bacteria 



In general, aerobic bacteria oxidize sugars to carbon dioxide and water, 

 but there are certain bacteria that are exceptions to this rule. Acetobacter 

 suboxydans, for example, oxidizes the carbons along the chain of a 

 polyhydroxy substance like glucose, but cannot cut the chain into shorter 

 pieces. From glucose it forms gluconic acid and 5-ketogluconic acid, 

 and from sorbitol it makes sorbose. The oxidizability of a compound 

 is very specific and depends upon the structure of the molecule. For 

 example, sorbitol and mannitol are oxidized to the corresponding keto- 

 sugars, sorbose and fructose, respectively; but ducitol, the alcohol cor- 

 responding to galactose, is not attacked. From a study of these and 

 other sugar alcohols, Bertrand concluded that two alcohol groups adjacent 

 to the primary alcohol must be cis to one another for oxidation to take 

 place. Dulcitol and xylitol do not have such a structure and are not 

 oxidized. However, as more polyhydric alcohols and bacteria have been 

 studied, it has been found that the requirements are both less specific 

 and more complex than is expressed by Bertrand's rule. 



Aerobic bacteria are very important in the production of vinegar, anti- 

 biotics, and enzymes; in the retting of flax; and in the disposal of sewage 

 by the activated sludge process. Antibiotics are of special interest and 

 recent development. An antibiotic is generally defined as an organic 

 compound produced by microorganisms which in small concentrations 

 inhibits or kills other microorganisms, usually pathogenic in character. 

 The definition is admittedly arbitrary, as it excludes inhibitory com- 

 pounds produced by higher plants, such as quinine, and purely synthetic 

 compounds, such as sulfa drugs. The term as thus defined, however, is 

 useful and convenient for practical purposes. 



The number of antibiotics reported in the literature runs into several 

 hundreds. Most of these are poorly characterized chemically, but ap- 

 proximately 60 have been obtained sufficiently pure to permit determina- 



