Fundamentally, bacterial metabolism involves all the cellular processes required for the organism’s survival and replication. Familiarity with bacterial metabolism is essential for understanding bacterial interactions with human host cells, the mechanisms bacteria use to cause disease, and the basis of diagnostic microbiology; that is, the tests and strategies used for laboratory identification of infectious etiologies. Because metabolism is an extensive and complicated topic, this section focuses on processes typical of medically relevant bacteria.

For the sake of clarity, metabolism is discussed in terms of four primary, but interdependent, processes: fueling, biosynthesis, polymerization, and assembly (Figure 1).

Fig1. Overview of bacterial metabolism, which includes the processes of fueling, biosynthesis, polymerization, and assembly. (Modified from Niedhardt FC, Ingraham JL, Schaechter M, editors: Physiology of the bacterial cell: a molecular approach, Sunderland, Mass,  1990, Sinauer Associates.)

FUELING

 Fueling is considered the utilization of metabolic path ways involved in the acquisition of nutrients from the environment, production of precursor metabolites, and energy production.

Acquisition of Nutrients

Bacteria use various strategies for obtaining essential nutrients from the external environment and transporting these substances into the cell’s interior. For nutrients to be internalized, they must cross the bacterial cell wall and membrane. These complex structures help protect the cell from environmental insults, maintain intracellular equilibrium, and transport substances into and out of the cell. Although some key nutrients (e.g., water, oxygen, and carbon dioxide) enter the cell by simple diffusion across the cell membrane, the uptake of other substances is controlled by membrane-selective permeability; still other substances use specific transport mechanisms.

Active transport is among the most common methods used for the uptake of nutrients such as certain sugars, most amino acids, organic acids, and many inorganic ions. The mechanism, driven by an energy-dependent pump, involves carrier molecules embedded in the mem brane portion of the cell structure. These carriers combine with the nutrients, transport them across the membrane, and release them inside the cell. Group translocation is another transport mechanism that requires energy but differs from active transport in that the nutrient being transported undergoes chemical modification. Many sugars, purines, pyrimidines, and fatty acids are transported by this mechanism.

Production of Precursor Metabolites

Once inside the cell, many nutrients serve as the raw materials from which precursor metabolites for subsequent biosynthetic processes are produced. These metabolites, listed in Figure 1, are produced through three central pathways; the Embden-Meyerhof-Parnas (EMP) pathway, the tricarboxylic acid (TCA) cycle, and the pentose phosphate shunt. These pathways and their relationship to one another are shown in Figure 2; not shown are the several alternative pathways (e.g., the Entner-Douder off pathway) that play key roles in redirecting and replenishing the precursors as they are used in subsequent processes.

Fig2. Overview diagram of the central metabolic pathways (Embden-Meyerhof-Parnas [EMP], the tricarboxylic acid [TCA] cycle, and  the pentose phosphate shunt). Precursor metabolites (see also Figure 1) that are produced are highlighted in red; production of energy  in the form of ATP (~P) by substrate-level phosphorylation is highlighted in yellow; and reduced carrier molecules for transport of electrons  used in oxidative phosphorylation are highlighted in green. (Modified from Niedhardt FC, Ingraham JL, Schaechter M, editors: Physiology of the bacterial cell: a molecular approach, Sunderland, Mass, 1990, Sinauer Associates.)

The production efficiency of a bacterial cell resulting from these precursor-producing pathways can vary substantially, depending on the growth conditions and availability of nutrients. This is an important consideration because the accurate identification of medically important bacteria often depends heavily on methods that measure the presence of products and byproducts of these metabolic pathways.

Energy Production

 The third type of fueling pathway is one that produces energy required for nearly all cellular processes, including nutrient uptake and precursor production. Energy production is accomplished by the breakdown of chemical substrates (i.e., chemical energy) through the degradative process of catabolism coupled with oxidation reduction reactions. In this process, the energy source molecule (i.e., substrate) is oxidized as it donates electrons to an electron-acceptor molecule, which is then reduced. The transfer of electrons is mediated through carrier molecules, such as nicotinamide-adenine dinucleotide (NAD+) and nicotinamide-adenine dinucleotide-phosphate (NADP+). The energy released by the oxidation-reduction reaction is transferred to phosphate-containing compounds, where high-energy phosphate bonds are formed. ATP is the most common of such molecules. The energy contained in this com pound is eventually released by the hydrolysis of ATP under controlled conditions. The release of this chemical energy, coupled with enzymatic activities, specifically catalyzes each biochemical reaction in the cell and drives nearly all cellular reactions.

The two general mechanisms for ATP production in bacterial cells are substrate-level phosphorylation and electron transport, also referred to as oxidative phosphorylation. In substrate-level phosphorylation, high-energy phosphate bonds produced by the central pathways are donated to adenosine diphosphate (ADP) to form ATP (see Figure 2). Additionally, pyruvate, a primary intermediate in the central pathways, serves as the initial substrate for several other pathways to generate ATP by substrate level phosphorylation. These other pathways constitute fermentative metabolism, which does not require oxygen and produces various end products, including alcohols, acids, carbon dioxide, and hydrogen. The specific fermentative pathways and the end products produced vary with different bacterial species. Detection of these products is an important basis for laboratory identification of bacteria.

Oxidative Phosphorylation. Oxidative phosphorylation involves an electron transport system that conducts a series of electron transfers from reduced carrier molecules such as NADH2 and NADPH2, produced in the central pathways (see Figure 2), to a terminal electron acceptor. The energy produced by the series of oxidation reduction reactions is used to generate ATP from ADP. When oxidative phosphorylation uses oxygen as the terminal electron acceptor, the process is known as aerobic respiration. Anaerobic respiration refers to processes that use final electron acceptors other than oxygen.

A knowledge of which mechanisms bacteria use to generate ATP is important for designing laboratory protocols for cultivating and identifying these organisms. For example, some bacteria depend solely on aerobic respiration and are unable to grow in the absence of oxygen (strictly aerobic bacteria). Others can use either aerobic respiration or fermentation, depending on the availability of oxygen (facultative anaerobic bacteria). For still others, oxygen is absolutely toxic (strictly anaerobic bacteria).

BIOSYNTHESIS

The fueling reactions essentially bring together all the raw materials needed to initiate and maintain all other cellular processes. The production of precursors and energy is accomplished through catabolic processes and the degradation of substrate molecules. The three remaining pathways for biosynthesis, polymerization, and assembly depend on anabolic metabolism. In anabolic metabolism, precursor compounds are joined for the creation of larger molecules (polymers) required for assembly of cellular structures (see Figure 1).

Biosynthetic processes use the precursor products in dozens of pathways to produce a variety of building blocks, such as amino acids, fatty acids, sugars, and nucleotides (see Figure 1). Many of these pathways are highly complex and interdependent, whereas other pathway are completely independent. In many cases, the enzymes that drive the individual pathways are encoded on a single mRNA molecule that has been transcribed from contiguous genes in the bacterial chromosome (i.e., an operon).

As previously mentioned, bacterial genera and species vary extensively in their biosynthetic capabilities. Know ledge of these variations is necessary to use optimal conditions for growing organisms under laboratory conditions. For example, some organisms may not be capable of synthesizing an essential amino acid necessary as a building block for proteins. Without the ability to synthesize the amino acid, the bacterium must obtain the building block from the environment. Similarly, if the organism is cultivated in the microbiology laboratory, the amino acid must be provided in the culture medium.

POLYMERIZATION AND ASSEMBLY

Various anabolic reactions assemble (polymerize) the building blocks into macromolecules, including lipids, lipopolysaccharides, polysaccharides, proteins, and nucleic acids. This synthesis of macromolecules is driven by energy and enzymatic activity in the cell. Similarly, energy and enzymatic activities also drive the assembly of various macromolecules into the component structures of the bacterial cell. Cellular structures are the product of all the genetic and metabolic processes discussed.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة تشهد تشييع جثمان المحقق الشيخ محمد رضا المامقاني

قسم الشؤون الفكرية يختتم برنامج (فتية سيد الماء) لرعاية الأيتام بنسخته الثانية

لتطوير المهارات الإدارية.. العتبة العباسية المقدسة تطلق الدورة التعليمية الأولى لمسؤولي وحداتها

قسم التطوير يقيم دورة تدريبية عن الظهور الإعلامي لملاكاته

المزيد

الآخبار الصحية

دراسة تكشف "السبب الخفي" وراء ترك التمارين الرياضية

كشف التوقيت "المثالي" لتغيير فرشاة الأسنان

بلاستيك في أنسجة بشرية.. دراسة تكشف مفاجأة طبية

دراسة تحذر من الإفراط في ممارسة الرياضة.. كيف يضر دماءك؟

المزيد

الآخبار التكنلوجية

"ديب سيك" تحجب أحدث نماذجها عن شركات الرقائق الأميركية

"سامسونغ" تطلق سلسلة هواتف "غالاكسي إس 26"

حملات سرية ورسائل تشويه.. كيف يتم استغلال "تشات جي بي تي"؟

أثر نفسي غير متوقع للرياضة.. مرتبط بالغضب

المزيد

مواضيع ذات صلة

Bacterial Genetics: Nucleic Acid Structure and Organization

Detection of Bacteremia: Culture Techniques

Detection of Bacteremia: Types of Blood Culture Bottle

Detection of Bacteremia: Specimen Collection

The Study of the Intestinal Microbiota

Chlamydia psittaci and Psittacosis

Chlamydia Pneumonia and Respiratory Infections

Chlamydia trachomatis Genital Infections and Inclusion Conjunctivitis

Trachoma

Coxiella burnetii

Ehrlichia and Anaplasma

Rickettsia and Orientia