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Medical Microbiology Lecture 5 Third class/ Dentistry College The Physiology of Microbial Metabolism Metabolism is the sum total of all chemical reactions occurring in the cell, can be divided into: catabolism and anabolism. In catabolism, molecules are reduced in complexity and free energy is made available. Anabolism involves the use of free energy to increase the complexity of molecules. Catabolic Reactions Organic nutrient substrates are catabolized in a wide variety of enzymatic processes that can be schematically divided into four phases: 1- Digestion. Bacterial exoenzymes split up the nutrient substrates into smaller molecules outside the cell. The exoenzymes represent important pathogenicity factors in some cases. 2- Uptake. Nutrients can be taken up by means of passive diffusion or, more frequently, specifically by active transport through the membrane(s). 3- Preparation for oxidation. Splitting off of carboxyl and amino groups, phosphorylation, etc. 4- Oxidation. This process is defined as the removal of electrons and H+ ions. The substance to which the H2 atoms are transferred is called the hydrogen acceptor. The two 1 Medical Microbiology Lecture 5 Third class/ Dentistry College basic forms of oxidation are defined by the final hydrogen acceptor, include the following: 4-1- Respiration. In aerobic respiration oxygen is the hydrogen acceptor. In anaerobic respiration, the hydrogen acceptor is a component of an inorganic salt. 4-2- Fermentation. Here an organic compound serves as the hydrogen acceptor. The main difference between fermentation and respiration is the energy yield, which can be greater from respiration than from fermentation for a given nutrient substrate. Fermentation processes involving microorganisms are designated by the final product, e.g., alcoholic fermentation, butyric acid fermentation, etc. The role of oxygen. Human pathogenic bacteria are classified according to their O2 requirements and tolerance : 1- Facultative anaerobes. These bacteria can oxidize nutrient substrates by means of both respiration and fermentation. 2- Obligate aerobes. These bacteria can only reproduce in the presence of O2. Aerobic organisms possess cytochromes and cytochrome oxidase, which are involved in the process of oxidative phosphorylation. Oxygen serves as the terminal electron acceptor in the sequence and water is one of the resultant products of respiration. Some of the oxidation–reduction enzymes interact with molecular oxygen to give rise to superoxide (•O2−), hydroxyl radicals (OH•), and hydrogen peroxide (H2O2), all of which are extremely toxic: O2 + e− (oxidative enzyme) −−−−−−−→ O2− O2 + H2O2( Nonenzymatic) −−−−−−→ O2 + OH• + OH− Hydrogen peroxide and the highly reactive superoxide anion must undergo further conversion immediately by superoxide dismutase and catalase are present in aerobic organisms and those that are aerotolerant, but not in strict anaerobes. which can eliminate the hydrogen peroxide formed: 2H2O2 −−−→ 2H2O + O2 3- Obligate anaerobes: many bacteria have electron transport chains that can operate with exogenous electron acceptors other than O2. As noted earlier, this energy-yielding 2 Medical Microbiology Lecture 5 Third class/ Dentistry College process is called anaerobic respiration. The major electron acceptors are nitrate, sulfate, and CO2.These bacteria die in the presence of O2. 4- Aero tolerant anaerobes. These bacteria oxidize nutrient substrates without using elemental oxygen although, unlike obligate anaerobes, they can tolerate it. Anabolic Reactions Microbial growth requires the polymerization of biochemical building blocks into proteins, nucleic acids, polysaccharides, and lipids. The building blocks must come preformed in the growth medium or must be synthesized by the growing cells. Respiration Compared to fermentation as a means of oxidizing organic compounds, respiration is a lot more complicated. Respirations result in the complete oxidation of the substrate by an outside electron acceptor. included: Glycolysis : is located in the cytoplasmic matrix of procaryotes and eucaryotes. The pathway as a whole may be divided into two parts In the initial six-carbon stage, glucose is phosphorylated twice and eventually converted to fructose 1,6- bisphosphate. This preliminary stage does not yield energy; in fact, two ATP molecules are expended for each glucose. The three-carbon stage of glycolysis begins when the enzyme fructose 1,6bisphosphate aldolase catalyzes the cleavage of fructose 1,6-bisphosphate into two halves, each with a phosphate group. One of the products, glyceraldehyde 3-phosphate, is converted directly to pyruvate in a five-step process. and a phosphate is simultaneously incorporated to give a high-energy molecule by substrate level phosphorylation process. Glucose + 2ADP + 2Pi +2NAD+-------------- 2 pyruvate + 2ATP+ 2NADH + 2H+ The tricarboxylic acid (TCA) cycle (also known as the citric acid cycle or the Kreb's cycle): when an organic compound is utilized as a substrate, the TCA cycle is used for the complete oxidation of the substrate. The end product that always results from the complete oxidation of an organic compound is CO2. 3 Medical Microbiology Lecture 5 Third class/ Dentistry College Electron transport system (ETS). The ETS is a sequence of electron carriers in the plasma membrane that transports electrons taken from the substrate through the chain of carriers to a final electron acceptor. The electrons enter the ETS at a very low redox potential and exit at a relatively high redox potential. This drop in potential releases energy that can be harvested by the cells in the process of ATP synthesis by the mechanisms of electron transport phosphorylation. The operation of the ETS establishes a proton motive force (pmf) due to the formation of a proton gradient across the membrane. Transmembranous ATPase enzyme (ATP synthetase). This enzyme utilizes the proton motive force established on the membrane (by the operation of the ETS) to synthesize ATP in the process of electron transport phosphorylation. The reaction catalyzed by the ATPase enzyme is ADP + Pi + 2 H+ <----------> ATP. References: 1-Prescott L. M., Microbiology, 5th Edition ,(2002). 2- Kayser, F.H., Medical Microbiology ( 2005). 3- Jawetz, Medical Microbiology ( 2007). 4- Todar, Medical Micro. Lecturer Zuhair S. Al Sehlawi For further informations please visit my web site on Dent.kuiraq.com 4