Evaluations of some phases of microbial nutrition, including nutrition essentials, sources of nutrients, and variations in requirements for different microorganisms, and relative amounts of principal elements in living cell have been presented in preceding chapter.
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Necessities of carbon, which forms the framework of all living and energy, which is essential for all life processes, were briefly presented. The purpose of all nutrients is to supply organisms with life essentials.
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Nutrients are useless without enzymes, which can break them down, either outside or inside cells, and eventually incorporate them into cell substances and, in the process, furnish energy for involved cells.
The incorporation of nutrients into cell substances and all necessary inter conversions involved in the process constitute one phase of metabolism.
Three phases of metabolism may be considered, although they cannot be clearly delineated, and the overall picture should be kept in mind when evaluating either of them.
The first phase of metabolism relates to gross compounds and the abilities of microorganisms to convert them into utilizabie units. This chapter will outline processes whereby some macromolecules are converted to smaller and utilizabie units.
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The following chapter will deal primarily with methods by which microorganisms obtained energy. These methods constitute the second phase. As will become evident, however, the two concepts cannot be separated, since energy is involved in all metabolic transformations.
The third phase of metabolism concerns the rebuilding of macromolecules by microorganisms. Biosynthesis of macromolecules always requires micromolecules and energy for building, however, and the third phase of metabolism will be closely related to the first two phases. Finally, one must consider waste products given off by living cells.
If nutrients are present and there is no means of eliminated of waste products, growth environment soon becomes toxic. This consideration becomes extremely important in an overall ecological view.
A separate chapter will not be devoted to this phase of metabolism, but final products will be discussed in the consideration of metabolic type. A number of fermentative products and some produced by respiration will be described in this chapter; parts of most following chapters, including those relative to pathology, will describe the final metabolic products of microorganisms.
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The necessity of inorganic (mineral) substances in microbial nutrition was presented somewhere, and roles of vitamins as coenzymes were also outlined.
The students should be keeping in mind the roles of principal elements and vitamins as he seeks to visualize processes involved in metabolism.
Some elements have been found to work chiefly as activators of enzymes in metabolism. Vitamins will be studied because they are molecular structures around which metabolic coenzymes are built.
Many steps are involved in carbohydrate fermentation, and the process depends on enzymes present in microorganisms.
Many steps, controlled by enzyme action, are involved in carbohydrate metabolism by any organism under any set of conditions. Process become more variable as conditions change and complications are rapidly multiplied where different organisms are involved.
In spite of many detailed differences, however, there are some overall patterns and principles that have wide applications. Established patterns of metabolism have not resulted from the investigation of any species or group of microorganisms but represent logical projections of data obtained from experiments involving a variety of forms.
Pieces of information were slowly gathered and fitted into a pattern until, at the present time; a large array of knowledge is available. More information concerning metabolism is constantly becoming available, and the student should bear in mind that, although many steps and general patterns are known, details of numerous processes are obscure.
The actions of microorganisms on carbohydrates may be considered fewer than three different headings, depending on the type of molecule attached, although there are many likenesses and overlaps. Any differentiation, therefore, is arbitrary and artificial.
Division is mostly for convenience in the treatment of subject matter, and overlaps occur. The student should attempt to obtain a view of the overall metabolic process instead of memorizing unrelated bits of information.
The first process is the fermentation of simple sugars, which is considered by some microbiologists according to the type of fermentation carried out by bacteria and other fungi. Major types include alcoholic, lactic, propionic, formic, butyl-butyric, and oxidative, but less obvious process also occur.
The type of fermentation in all except the oxidative process is named for the end product formed. The oxidative type, by its very name and connotation, is actually no fermentation at all, but is a process in which simple sugars, or their fermentative products, are broken down into active acids and other end products.
Other actions of microorganisms in carbohydrate breakdown include the hydrolysis of disaccharides and trisaccharides to simple sugars and the breakdown of polysaccharides. In addition to breakdown, microorganisms also carry on biosynthesis of macromolecular compounds.
The breakdown of sugars does not require the presence of oxygen for a final electron acceptor. By definition, the process is fermentative because in each case the final electron acceptor is an organic compound.
Early workers thought that fermentation was a chemical process instead of a biological process:
For many years fermentation was considered to be a chemical process. The nature of chemical changes that occurred as the result of the presence of living organisms, however, was not known by early observers.
Almost forty years after Pasteur’s discourse on the nature of fermentation, as accidental discovery by Buchner demonstrated the occurrence of cell-free fermentation by enzymes.
Other workers had postulated that ferments which resembled proteins, and which were present in living organisms, played a role in fermentation, but specific data were lacking.
In an attempt to preserve therapeutic yeast juice, produced by grinding yeast and sand, Buchner added sugar to the mixture. The sugar was fermented with the evolution of carbon dioxide and production of ethyl alcohol.
After Buchner’s discovery, numerous fermentative preparations of yeast juice were utilized as models by experiments in the field of fermentation.
A large number of preparations were found to ferment sugar, but reaction rates were only 2% to 10% as rapid as that produced by living yeast. Preparations of yeast juice or yeast cells would attach glucose, fructose, mannose, maltose, and sucrose.
Early experiments with fermentation discovered that during the process of sugar fermentation, yeast had a high reducing activity. Reduction could be demonstrated by actions on methylene blue, powdered sulfur, or thiosulfate. Decomposition of pyruvate to acetaldehyde and carbon dioxide was also demonstrated as follows:
Neuberg then postulated that yeast fermentation proceeded through pyruvate. He later discovered that the addition of sodium bisulfite to a fermenting yeast mixture precipitated out acetaldehyde and slowed the eventual production of the ethyl alcohol.
A corresponding increase in the amount of glycerol accompanied this reaction. For each mole of acetaldehyde precipitated, a mole of glycerol resulted. The hydrogen that would have reduced acetaldehyde to ethyl alcohol was now being utilized in glycerol production.
Sodium sulfite is still added to fermenting mixture in the commercial production of glycerol. Acids or alkalis, or their salts, as well as some other compounds, have been utilized for accomplishment of the same result.
The conversion of glucose to alcohol was originally thought, by Buchner, to result from the action of a single enzyme zymase, but other steps were later discovered.
The steps in fermentation conversions, and the role of phosphate in the process, were not understood until the formation of phosphate esters in both hexoses and trioses was demonstrated.