Bacterial Metabolism

Bacterial Metabolism

General Concepts

Heterotrophic Metabolism

Bacterial Metabolism heterotrophic metabolism is the biological oxidation of organic substances such as glucose to produce ATP and simpler organic (or inorganic) chemicals that the bacterial cell need for biosynthetic or assimilatory activities.


Respiration is a kind of heterotrophic metabolism that utilises oxygen and produces 380,000 calories from the oxidation of one mole of glucose. (Another 308,000 calories are wasted as heat.)


The terminal electron (or hydrogen) acceptor in fermentation, another kind of heterotrophic metabolism, is an organic substance rather than oxygen. This imperfect method of glucose oxidation produces less energy, but it promotes anaerobic development.

Krebs Cycle

The Krebs cycle is the oxidative mechanism in respiration that fully decarboxylates pyruvate (through acetyl coenzyme A). 15 moles of ATP (150,000 calories) are produced by the route.

Glyoxylate Cycle

The glyoxylate cycle, seen in some bacteria, is a variant of the Krebs cycle. The oxidation of fatty acids or other lipid molecules produces acetyl coenzyme A.

Electron Transport and Oxidative Phosphorylation

ATP is produced in the last stage of respiration by a series of electron transfer processes within the cytoplasmic membrane that drive the oxidative phosphorylation of ADP to ATP. For this process, bacteria utilise a variety of flavins, cytochrome and non-heme iron components, as well as several cytochrome oxidases.

Mitchell or Proton Extrusion Hypothesis

The Mitchell theory explains energy conservation in all cells by generating a proton motive force by the selective extrusion of H+ ions over a proton-impermeable membrane. This energy is required for ATP production in both respiration and photosynthesis.

Bacterial Photosynthesis

Bacterial photosynthesis is an anaerobic, light-dependent method of metabolism in which carbon dioxide is converted to glucose, which is then utilised for both biosynthesis and energy generation.. Depending on the hydrogen source used to reduce CO2, bacteria can undergo both photolithotrophic and photoorganotrophic reactions.


Autotrophy is a kind of metabolism that is exclusively present in bacteria. Inorganic substances (e.g., NH3, NO2-, S2, and Fe2+) are oxidised directly (without the use of sunlight) to produce energy. This metabolic mode, like photosynthesis, requires energy for CO2 reduction, but no lipid-mediated activities are involved. Chemotrophy, chemoautotrophy, and chemolithotrophy are all names for this metabolic state.

Anaerobic Respiration

Another heterotrophic method of metabolism is anaerobic respiration, which uses a molecule other than O2 as a terminal electron acceptor. Acceptor chemicals for methane-producing bacteria include NO3-, SO42-, fumarate, and even CO2.

The Nitrogen Cycle

The nitrogen cycle is a recycling process that involves bacteria, plants, and mammals using organic and inorganic nitrogen molecules metabolically and recycling them. Bacterial photosynthesis is a light-dependent, anaerobic method of metabolism in which carbon dioxide is converted to glucose, which is used for both biosynthesis and energy generation.

Bacterial Metabolism


Metabolism encompasses all biological events that occur within a cell or organism. The chemical variety of substrate oxidations and dissimilation reactions (reactions that break down substrate molecules), which are used to create energy in bacteria, is the subject of bacterial metabolism research.. The study of the intake and utilisation of inorganic or organic substances essential for growth and the maintenance of a cellular steady state (assimilation processes) is also included in the scope of bacterial metabolism. These exergonic (energy-producing) and endergonic (energy-requiring) processes are catalysed within the live bacterial cell by integrated enzyme systems, resulting in cell self-replication. Microbial cells’ capacity to exist, operate, and proliferate in an appropriate chemical environment (such as a bacterial environment).

Heterotrophic Metabolism

All pathogens are heterotrophic bacteria that get their energy from oxidation of organic molecules. The most typically oxidised substances are carbohydrates (especially glucose), lipids, and protein. Bacterial oxidation of these organic molecules leads in the creation of ATP as a chemical energy source. This mechanism also allows for the production of simpler organic compounds (precursor molecules) required by bacteria cells for biosynthetic or assimilatory processes.

The intermediate chemicals of the Krebs cycle operate as precursor molecules (building blocks) for the energy-demanding production of complex organic compounds in bacteria. Amphibolic degradation processes create energy while also producing precursor molecules for the production of new cellular components.

Preformed organic molecules are required by all heterotrophic bacteria. These carbon- and nitrogen-containing molecules are growth substrates that may be utilised aerobically or anaerobically.


The most commonly utilised substrate for researching heterotrophic metabolism is glucose. The following chemical equation describes how most aerobic organisms totally oxidise glucose:

This equation describes the process of cellular oxidation known as respiration. Respiration happens within plant and animal cells, producing 38 ATP molecules (as energy) from the oxidation of one glucose molecule. This produces around 380,000 calories (cal) per mode of glucose (ATP 10,000 cal/mole). Thermodynamically, one mole of glucose should generate roughly 688,000 cal; the energy not preserved physiologically as chemical energy (or ATP production) is liberated as heat (308,000 cal). As a result, the cellular respiratory mechanism is only around 55% efficient.

The most studied dissimilatory process leading to energy generation or ATP synthesis is glucose oxidation. Three basic metabolic processes may be involved in the full oxidation of glucose. The glycolytic or Embden pathway is the first.Meyerhof-Parnas –

Respiration occurs when any organic substance (typically a carbohydrate) is entirely oxidised to CO2 and H2O. In aerobic respiration, molecular O2 acts as an electron terminal acceptor. Depending on the bacteria researched, NO3-, SO42-, CO2, or fumarate can function as terminal electron acceptors (rather than O2) in anaerobic respiration. Bacterial metabolism study focuses on the chemical variety of substrate oxidations and dissimilation reactions (reactions that break down substrate molecules), which are used to create energy in bacteria. Ammonia is also produced when protein (or an amino acid) is oxidised.

Bacteria differ from cyanobacteria (blue-green algae) and eukaryotes in that glucose oxidation can occur via many pathways. Glycolysis is one of numerous routes by which bacteria can catabolically destroy glucose. In bacteria and yeasts, the glycolytic pathway is most typically connected with anaerobic or fermentative metabolism. Other minor heterofermentative routes, such as the phosphoketolase pathway, exist in bacteria.

Furthermore, bacteria have two alternative glucose-catabolizing pathways: the oxidative pentose phosphate route (hexose monophosphate shunt) (Fig. 4-3) and the Entner-Doudoroff system, which is almost exclusively found in obligate aerobic bacteria (Fig. 4-4). Because these organisms lack the enzyme phosphofructokinase and hence employ the Entner-Doudoroff pathway for glucose catabolism, the highly oxidative Azotobacter and most Pseudomonas species, for example, use the Entner-Doudoroff pathway for glucose catabolism.

The hexose monophosphate shunt also contributes to glucose dissimilation (Fig. 4-3). In the presence of two glycolytic pathway inhibitors (iodoacetate and fluoride), an oxidative route was revealed in tissues that actively metabolise glucose. Neither inhibitor had an impact on glucose dissimilation, and NADPH + H+ was generated directly from glucose-6-phosphate dehydrogenase oxidation (to 6-phosphoglucono–lactone). Following that, the pentose phosphate route allows for direct oxidative decarboxylation of glucose to pentoses. This oxidative metabolic system’s capacity to bypass glycolysis explains the word shunt.

metabolic properties

The Entner-Doudoroff pathway’s metabolic steps are similar to the hexose monophosphate shunt, except that pentose sugars are not directly produced. The two routes are similar until 6-phosphogluconate is formed (see Fig. 4-4), at which point they diverge.

Several metabolic properties are shared by all main routes of glucose or hexose catabolism. First, there are the preliminary procedures that produce critical intermediate chemicals such triose-PO4, glyceraldehyde-3-phosphate, and/or pyruvate. The latter two chemicals are nearly always necessary for additional assimilatory or dissimilatory events to occur within the cell. Second, ATP, not inorganic phosphate (Pi), is the primary source of phosphate in all processes involving the phosphorylation of glucose or other hexoses. Actually, the chemical energy contained in ATP must first be used in the first phase of glucose metabolism (through kinase-type enzymes) to form glucose-6-phosphate, which activates the hexose catabolic processes. Third, NADH + H+ or NADPH + H+ is directly created as reducing equivalents (potential energy) by one or more of the enzymatic processes involved in the reaction.


Another example of heterotrophic metabolism is fermentation, which requires an organic substance as a terminal electron (or hydrogen) acceptor. Simple organic end products are generated in fermentations as a result of the anaerobic dissimilation of glucose (or another substance). The dehydrogenation mechanisms that occur during the enzymatic breakdown of glucose provide energy (ATP).. The simple organic byproducts of this imperfect biologic oxidation process also function as final electron and hydrogen acceptors. These organic end products are released into the media as waste metabolites (typically alcohol or acid) after reduction. Bacterial partial oxidation of organic substrate molecules yields adequate energy for microbial development. The most frequent hexose utilised to examine fermentation processes is glucose.

Pasteur showed fermentation in the late 1850s.

In the studies reported, yeast fermentation is demonstrated to be a direct result of the processes of feeding, absorption, and life when they are carried out in the absence of free oxygen. The heat necessary to do that job has to have been derived from the fermenting matter’s disintegration…. Fermentation by yeast appears to be fundamentally linked to the ability held by this minute cellular plant of conducting its respiratory duties, in some way, using the oxygen present in sugar.

Most microbial fermentations dissipate glucose via the glycolytic route (Fig. 4-1). Pyruvate or a molecule formed enzymatically from pyruvate, such as acetaldehyde, -acetolactate, acetyl SCoA, is the most often synthesised simple organic chemical.

Krebs Cycle

The Krebs cycle (also known as the tricarboxylic acid cycle or the citic acid cycle) is an oxidative respiration mechanism that fully decarboxylates pyruvate or acetyl SCoA to CO2. This process happens in bacteria via acetyl SCoA, which is the initial result of pyruvate dehydrogenase’s oxidative decarboxylation.

If 2 pyruvate molecules are produced from the dissimilation of 1 glucose molecule, a total of 30 ATP molecules are produced. All CO2 molecules produced during the respiratory process are accounted for by the decarboxylation of pyruvate, isocitrate, and -ketoglutarate. The enzymatic processes in the Krebs cycle are shown in Figure 4-6. The reduced molecules produced by the Krebs cycle (NADH + H+, NADPH + H+, and succinate) contain the chemical energy preserved by the Krebs cycle. Until the last phase of respiration (electron transport and oxidative phosphorylation), the potential energy contained in these reduced molecules is not accessible as ATP.

As a result, the Krebs cycle represents yet another preliminary stage in the respiratory process. If 1 molecule of pyruvate is completely oxidised to 3 molecules of CO2, producing 15 ATP molecules, 1 molecule of glucose can be oxidised to produce up to 38 ATP molecules if glucose is dissimilated by glycolysis and the Krebs cycle (assuming that the electron transport/oxidative phosphorylation reactions are bioenergetically identical to those of eukaryotic mitochondria).

Glyoxylate Cycle

The Krebs cycle acts similarly in bacteria and eukaryotic systems in general, however there are significant variances across bacteria. One distinction is that in obligate aerobes, L-malate may be directly oxidised by molecular O2 through an electron transport chain. Because -ketoglutarate dehydrogenase is absent in other bacteria, only some Krebs cycle intermediate processes occur.

A variation of the Krebs cycle known as the glyoxylate cycle or shunt (Fig. 4-7) seen in some bacteria. This shunt works in the same way as the Krebs cycle, although it lacks several of the Krebs cycle enzyme processes. The glyoxylate cycle is largely an oxidative mechanism in which acetylSCoA is produced by the oxidation of acetate, which is typically produced through the oxidation of fatty acids. The conversion of fatty acids into

Electron Transport and Oxidative Phosphorylation

Electron Transport and Oxidative Phosphorylation

The last step of respiration involves a sequence of oxidation-reduction electron transfer processes that create the energy needed to drive oxidative phosphorylation, which produces ATP. Electron transport and oxidative phosphorylation enzymes are found on the bacterial inner (cytoplasmic) membrane. This membrane is invaginated to create structures known as respiratory vesicles, lamellar vesicles, or mesosomes, which serve as the bacterial analogue of the mitochondrial membrane.

Bacterial Metabolism Respiratory electron transport chains differ widely across bacteria and are missing in others. NADH + H+, NADPH + H+, and succinate (as well as coacylated fatty acids such as acetylSCoA) are oxidised via the respiratory electron transport chain of eukaryotic mitochondria. These chemicals are also oxidised through the bacterial electron transport chain, although they can also be oxidised directly by non-pyridine nucleotide-dependent mechanisms.

Cytochrome oxidases are often found in bacteria as combinations of a1: d: o and a + a3:o. Bacteria also have mixed-function oxidases such cytochromes P-450 and P-420, as well as cytochromes c’ and c’c’ that react with carbon monoxide. These various forms of oxygen-reactive cytochromes have definite evolutionary relevance. Bacteria existed before O2 was generated; when O2 became available as a metabolite, bacteria evolved to use it in a variety of ways, which explains the diversity of bacterial oxygen-reactive hemoproteins.

The Bacterial Metabolism oxidase reaction, which separates Gram-negative organisms into two primary categories, oxidase positive and oxidase negative, is used to study cytochrome oxidases in many harmful bacteria. This oxidase reaction is confirmed by oxidising N,N,N’, N’-tetramethyl-p-phenylenediamine (to Wurster’s blue) or synthesising indophenol blue..

Mitchell or Proton Extrusion Hypothesis Bacterial Metabolism

The chemiosmosis connection of oxidative and photosynthetic phosphorylation’s, often known as the Mitchell hypothesis, is a difficult yet appealing proposal for explaining energy conservation in biological systems. This theory seeks to explain free energy conservation in this process using an osmotic potential induced by a proton concentration differential (or proton gradient) across a proton-impermeable membrane. During membrane-bound electron transport, energy is created by a proton extrusion process, which acts as a proton pump; energy conservation and coupling follow. This is an unavoidable “intact” membrane phenomena. The energy thus preserved is connected to ATP production (again within the boundaries of the membrane). This would happen in all biological organisms, even lactic acid bacteria, which lack a membrane.

Mitchell’s hypothesis is complicated, and numerous changes have been made, but the theory’s most appealing characteristic is that it integrates all bioenergetic conservation principles into a single idea that requires an unbroken membrane vesicle to work correctly. Figure 4-9 depicts how the Mitchell hypothesis may be used to explain energy creation, conservation, and transfer as a result of a coupling process. The least appealing feature of the chemiosmotic hypothesis is the lack of knowledge of how chemical energy is really conserved inside the membrane and conveyed through coupling for ATP production.

Bacterial Photosynthesis

Bacterial Metabolism is seen in many prokaryotes (bacteria and cyanobacteria) (Table 4-1). The two classes of prokaryotes’ photosynthesis differ primarily in the type of molecule that serves as the hydrogen donor in the conversion of CO2 to glucose (Table 4-1). Phototrophic creatures vary from heterotrophic organisms in that they use intracellularly synthesised glucose for biosynthetic reasons (as in starch synthesis) or for energy production (by cellular respiration).

Unlike phototrophs, heterotrophs require an outside supply of glucose (or another preformed organic component) as a substrate. Heterotrophs are unable to produce considerable amounts of glucose from CO2 by employing H2O or (H2S) as a hydrogen source and sunlight as an energy source. Plant metabolism is a prime example.

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