Download Medical Microbiology Lecture 5 Third class/ Dentistry College The

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
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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
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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.
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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
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