Unsaturated Fatty Acids
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Biotin and Lipoic Acid: Synthesis, Attachment, and Regulation
- Author: John E. Cronan
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Citation: Cronan J. 2014. Biotin and Lipoic Acid: Synthesis, Attachment, and Regulation, EcoSal Plus 2014; doi:10.1128/ecosalplus.ESP-0001-2012
- DOI 10.1128/ecosalplus.ESP-0001-2012
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Two vitamins, biotin and lipoic acid, are essential in all three domains of life. Both coenzymes function only when covalently attached to key metabolic enzymes. There they act as “swinging arms” that shuttle intermediates between two active sites (= covalent substrate channeling) of key metabolic enzymes. Although biotin was discovered over 100 years ago and lipoic acid 60 years ago, it was not known how either coenzyme is made until recently. In Escherichia coli the synthetic pathways for both coenzymes have now been worked out for the first time. The late steps of biotin synthesis, those involved in assembling the fused rings, were well described biochemically years ago, although recent progress has been made on the BioB reaction, the last step of the pathway in which the biotin sulfur moiety is inserted. In contrast, the early steps of biotin synthesis, assembly of the fatty acid-like “arm” of biotin were unknown. It has now been demonstrated that the arm is made by using disguised substrates to gain entry into the fatty acid synthesis pathway followed by removal of the disguise when the proper chain length is attained. The BioC methyltransferase is responsible for introducing the disguise, and the BioH esterase is responsible for its removal. In contrast to biotin, which is attached to its cognate proteins as a finished molecule, lipoic acid is assembled on its cognate proteins. An octanoyl moiety is transferred from the octanoyl acyl carrier protein of fatty acid synthesis to a specific lysine residue of a cognate protein by the LipB octanoyltransferase followed by sulfur insertion at carbons C-6 and C-8 by the LipA lipoyl synthetase. Assembly on the cognate proteins regulates the amount of lipoic acid synthesized, and, thus, there is no transcriptional control of the synthetic genes. In contrast, transcriptional control of the biotin synthetic genes is wielded by a remarkably sophisticated, yet simple, system, exerted through BirA, a dual-function protein that both represses biotin operon transcription and ligates biotin to its cognate proteins.
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Biosynthesis of Membrane Lipids
- Authors: John E. Cronan, Jr., and Charles O. Rock
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Citation: Cronan, Jr. J, Rock C. 2008. Biosynthesis of Membrane Lipids, EcoSal Plus 2008; doi:10.1128/ecosalplus.3.6.4
- DOI 10.1128/ecosalplus.3.6.4
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The pathways in Escherichia coli and (largely by analogy) S. enterica remain the paradigm of bacterial lipid synthetic pathways, although recently considerable diversity among bacteria in the specific areas of lipid synthesis has been demonstrated. The structural biology of the fatty acid synthetic proteins is essentially complete. However, the membrane-bound enzymes of phospholipid synthesis remain recalcitrant to structural analyses. Recent advances in genetic technology have allowed the essentialgenes of lipid synthesis to be tested with rigor, and as expected most genes are essential under standard growth conditions. Conditionally lethal mutants are available in numerous genes, which facilitates physiological analyses. The array of genetic constructs facilitates analysis of the functions of genes from other organisms. Advances in mass spectroscopy have allowed very accurate and detailed analyses of lipid compositions as well as detection of the interactions of lipid biosynthetic proteins with one another and with proteins outside the lipid pathway. The combination of these advances has resulted in use of E. coli and S. enterica for discovery of new antimicrobials targeted to lipid synthesis and in deciphering the molecular actions of known antimicrobials. Finally,roles for bacterial fatty acids other than as membrane lipid structural components have been uncovered. For example, fatty acid synthesis plays major roles in the synthesis of the essential enzyme cofactors, biotin and lipoic acid. Although other roles for bacterial fatty acids, such as synthesis of acyl-homoserine quorum-sensing molecules, are not native to E. coli introduction of the relevant gene(s) synthesis of these foreign molecules readily proceeds and the sophisticated tools available can used to decipher the mechanisms of synthesis of these molecules.
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Biotin and Lipoic Acid: Synthesis, Attachment, and Regulation
- Author: John E. Cronan
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Citation: Cronan J. 2008. Biotin and Lipoic Acid: Synthesis, Attachment, and Regulation, EcoSal Plus 2008; doi:10.1128/ecosalplus.3.6.3.5
- DOI 10.1128/ecosalplus.3.6.3.5
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Abstract:
Two vitamins, biotin and lipoic acid, are essential in all three domains of life. Both coenzymes function only when covalently attached to key metabolic enzymes. There they act as "swinging arms" that shuttle intermediates between two active sites (= covalent substrate channeling) of key metabolic enzymes. Although biotin was discovered over 100 years ago and lipoic acid was discovered 60 years ago, it was not known how either coenzyme is made until recently. In Escherichia coli the synthetic pathways for both coenzymes have now been worked out for the first time. The late steps of biotin synthesis, those involved in assembling the fused rings, were well described biochemically years ago, although recent progress has been made on the BioB reaction, the last step of the pathway, in which the biotin sulfur moiety is inserted. In contrast, the early steps of biotin synthesis, assembly of the fatty acid-like "arm" of biotin, were unknown. It has now been demonstrated that the arm is made by using disguised substrates to gain entry into the fatty acid synthesis pathway followed by removal of the disguise when the proper chain length is attained. The BioC methyltransferase is responsible for introducing the disguise and the BioH esterase for its removal. In contrast to biotin, which is attached to its cognate proteins as a finished molecule, lipoic acid is assembled on its cognate proteins. An octanoyl moiety is transferred from the octanoyl-ACP of fatty acid synthesis to a specific lysine residue of a cognate protein by the LipB octanoyl transferase, followed by sulfur insertion at carbons C6 and C8 by the LipA lipoyl synthetase. Assembly on the cognate proteins regulates the amount of lipoic acid synthesized, and thus there is no transcriptional control of the synthetic genes. In contrast, transcriptional control of the biotin synthetic genes is wielded by a remarkably sophisticated, yet simple, system exerted through BirA, a dual-function protein that both represses biotin operon transcription and ligates biotin to its cognate protein.
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The Cold Shock Response
- Authors: Sangita Phadtare, and Masayori Inouye
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Citation: Phadtare S, Inouye M. 2008. The Cold Shock Response, EcoSal Plus 2008; doi:10.1128/ecosalplus.5.4.2
- DOI 10.1128/ecosalplus.5.4.2
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This review focuses on the cold shock response of Escherichia coli. Change in temperature is one of the most common stresses that an organism encounters in nature. Temperature downshift affects the cell on various levels: (i) decrease in the membrane fluidity; (ii) stabilization of the secondary structures of RNA and DNA; (iii) slow or inefficient protein folding; (iv) reduced ribosome function, affecting translation of non-cold shock proteins; (v) increased negative supercoiling of DNA; and (vi) accumulation of various sugars. Cold shock proteins and certain sugars play a key role in dealing with the initial detrimental effect of cold shock and maintaining the continued growth of the organism at low temperature. CspA is the major cold shock protein of E. coli, and its homologues are found to be widespread among bacteria, including psychrophilic, psychrotrophic, mesophilic, and thermophilic bacteria, but are not found in archaea or cyanobacteria. Significant, albeit transient, stabilization of the cspA mRNA immediately following temperature downshift is mainly responsible for its cold shock induction. Various approaches were used in studies to detect cold shock induction of cspA mRNA. Sugars are shown to confer protection to cells undergoing cold shock. The study of the cold shock response has implications in basic and health-related research as well as in commercial applications. The cold shock response is elicited by all types of bacteria and affects these bacteria at various levels, such as cell membrane, transcription, translation, and metabolism.
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Genomic Prospecting for Microbial Biodiesel Production
- Authors: Athanasios Lykidis, Natalia Ivanova
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Source: Bioenergy , pp 407-418
Publication Date :
January 2008
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This chapter focuses on the structure and regulation of the pathways utilized by various microbes (bacteria, algae, and yeasts) for the production of fatty acids and triacylglycerols (TAGs). An important observation with regard to the possibility of microbial biodiesel production was made when fatty acyl-ACP thioesterase (FAT) enzymes were overproduced in Escherichia coli. Thus far it represents the most efficient way to uncouple fatty acid formation from phospholipid and membrane biosynthesis in E. coli. Phosphatidic acid (PtdOH) is a key branching point in de novo lipid metabolism, and it is converted either to CDP-diacylglycerol (DAG) or DAG depending on the organism. CDP-DAG and DAG serve as intermediates in membrane phospholipid biosynthesis and, in addition, DAG is converted to TAG. Phosphatidate phosphatases (PAPs) in coordination with phospholipid-producing enzymes are key regulators of the flux of carbon towards TAGs. PAPs catalyze the conversion of PtdOH to DAG; the primary destination of DAG is the synthesis of membrane phospholipids, whereas excess DAG is directed towards TAG. The emerging theme from genome comparisons underlines the evolution of distinct regulatory mechanisms in various phylogenetic groups. All free-living organisms have the machinery to synthesize fatty acids, and conceptually, they could be exploited for biodiesel production. However, the photosynthetic organisms provide the unique opportunity to couple CO2 sequestration to lipid accumulation and subsequent biodiesel production.
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Deep-Sea Geomicrobiology
- Authors: Jiasong Fang, Dennis A. Bazylinski
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Source: High-Pressure Microbiology , pp 237-264
Publication Date :
January 2008
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The rapid development and growth of geomicrobiology can partially be attributed to discoveries in the last several decades of unique extremophiles present in many different harsh environments that play key roles in the biogeochemistry that occurs in these environments. The development of new technologies and experimental approaches in geomicrobiology and in the studies of extremophiles has spawned a revolution that will surely have profound social and economic impact now and in the future. The chapter focuses on the piezophilic members of the extremophiles, with an emphasis on geomicrobiological considerations. The deep sea in general is an oligotrophic environment, except in areas of cold seeps and hydrothermal vents. The pressure-induced changes in fatty acid composition are comparable to those induced by temperature changes and that homeoviscous adaptation of membrane lipids occurs in piezophilic bacteria in response to pressure. The majority of the fatty acids were unsaturated, with one, five, or six double bonds. The biosynthesis of monounsaturated fatty acids was significantly inhibited (10 to 37%) by the addition of cerulenin, whereas the concentrations of polyunsaturated fatty acid (PUFA) increased two to four times. Lipids and stable carbon isotopes preserved in lipids have proven to be excellent biosignatures applied to deep-sea geomicrobiology. Fatty acid compositions of piezophilic bacteria are discussed in this chapter.
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14 Metabolic Pathways Relevant to Predation, Signaling, and Development
- Authors: Patrick D. Curtis, Lawrence J. Shimkets
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Source: Myxobacteria , pp 241-258
Publication Date :
January 2008
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The first portion of this chapter, entitled “Catabolic Pathways,” deals with the catabolism of amino acids and lipids, as they are the principal carbon and energy sources derived from prey bacteria. The second portion of the chapter, entitled “Anabolic Pathways,” highlights the synthesis of lipids because of their unusual chemical structures in myxobacteria, and also the spore-specific components trehalose and ether lipids. Most myxobacteria, including Myxococcus xanthus, can catabolize prey microorganisms. M. xanthus utilizes amino acids and lipids as carbon and energy sources, incorporates purines and pyrimidines via salvage pathways, and fails to utilize sugars. In most cases there is excellent agreement between the presence of a particular amino acid catabolic pathway and the ability of that amino acid to stimulate growth in defined and minimal media. Lipid oxidation has been demonstrated by 14C-labeling experiments in Myxococcus virescens and methyloleate feeding in M. xanthus. Fatty acids are usually degraded by β oxidation, where two carbon acetate units are sequentially removed from the carboxyl end of the fatty acid, also known as the Δ terminus, as opposed to the methyl end or ω terminus. Monosaccharides are used for exopolysaccharide, peptidoglycan, and lipopolysaccharide biosynthesis. In Escherichia coli, trehalose is degraded to glucose by a periplasmic trehalase; no homolog exists in M. xanthus.
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7 Antibiotics and New Inhibitors of the Cell Wall
- Authors: Lynn G. Dover, Luke Alderwick, Veemal Bhowruth, Alistair K. Brown, Laurent Kremer, Gurdyal S. Besra
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Source: The Mycobacterial Cell Envelope , pp 107-131
Publication Date :
January 2008
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A “golden age” of tuberculosis (TB) chemotherapy was heralded by the discovery of streptomycin in 1944. The chemotherapeutic regimen consists of an initial 2-month phase of treatment with isoniazid (INH), rifampin (RIF), pyrazinamide (PZA), and ethambutol (EMB), followed by a continuation phase of treatment lasting four months with INH and RIF. Important considerations for new agents include enhancement of penetration of infection sites, such as lung cavities, and long biological half-lives; achieving either might represent a significant advance toward shortening therapy and lead to simpler treatment regimens with improved patient compliance. The products of the emb locus of Mycobacterium avium were identified as the targets for EMB using a strategy of target overexpression. The locus contains three genes, embR, embA, and embB; the former encodes a putative regulator of embA and embB and is expendable for the resistant phenotype, which is copy number dependent. Pharmacoproteomic studies with M. tuberculosis H37Rv revealed that similar protein profiles were catalogued after both EMB and SQ109 treatments. A spontaneous mutant of Mycobacterium smegmatis designated mc2651 is resistant to INH, but retains wild-type KatG activity. Analyses of treated sensitive bacteria using electron microscopy revealed dysfunction in cell wall biosynthesis and incomplete septation. The increased abundance of CmaA2, involved in mycolic acid biosynthesis under anaerobic conditions suggests a level of metabolic activity related to mycolic acid biosynthesis under conditions usually associated with a transition to dormancy that may be linked, resulting in modulation of mycolic acid chain length during a dormant or persistent anaerobic state.
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Cell Properties and Processes
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Source: Molecular Biology and Biotechnology: A Guide for Teachers, Third Edition , pp 45-71
Publication Date :
January 2008
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This chapter focuses on the remarkable similarity of cells, whether the cell is a one-celled organism, such as a bacterium, or a highly specialized cell in a multicellular organism. All cells share certain basic features: they have molecular machinery for duplicating DNA and for breaking down and synthesizing molecules, they reproduce by dividing in two, they use the same molecular building blocks, and they are enclosed by a hydrophobic membrane that separates the cell from its surroundings. All biological phenomena are based on chemical interactions, so understanding the cellular processes described throughout the chapter requires a basic understanding of cell chemistry. Biotechnology is based on the use of living cells and their component parts. Therefore, knowing something about the structure and function of cells and the molecules they contain is essential to understanding biotechnology’s scientific foundations, potential applications, and possible limitations. All living cells carry out a number of essential processes that are the defining traits of life. Cells grow and reproduce, maintain their internal environments, respond to the external environment, and communicate with each other. Cellular processes can be reduced to a series of chemical reactions, most of which are catalyzed by enzymes. To carry out all of these processes, cells require a constant supply of energy. Finally, cells also regulate their processes to ensure they are carried out in an orderly and efficient way.
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Cell Properties and Processes
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Source: Molecular Biology and Biotechnology: A Guide for Students, Third Edition , pp 45-72
Publication Date :
January 2008
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This chapter focuses on the remarkable similarity of cells, whether the cell is a one-celled organism, such as a bacterium, or a highly specialized cell in a multicellular organism. The contribution of cell membranes to cellular organization extends beyond the structural organization they provide. Both the plasma and internal membranes are actively involved in selecting which molecules are allowed to pass through the membrane. Chemical analysis of cells revealed that different cells from different organisms had remarkably similar chemistries. In addition, scientists discovered that all cells contain four types of molecules-biological molecules-found only in living things: carbohydrates, lipids, proteins, and nucleic acids. Fats and oils have similar molecular makeups. They are made of glycerol, a three-carbon molecule, combined with different numbers and types of fatty acids. Fatty acids are long chains of carbon and hydrogen atoms that terminate in a carboxyl group. Like lipids, carbohydrates play important roles in structure and energy storage. Carbohydrates are the primary components of the cell walls of bacteria and plants, and the hard exoskeleton of insects is made of another carbohydrate, chitin. The activities that distinguish living organisms from non-living matter are extremely complicated but are remarkably similar in all organisms, from bacteria to humans. Cells in multicelled organisms communicate among themselves to coordinate their activities. External and internal conditions are continually assessed by cells, and this assessed information is transferred to other cells so that they can act on the information.
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Psychrophiles: Membrane Adaptations
- Author: Nicholas J. Russell
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Source: Physiology and Biochemistry of Extremophiles , pp 155-164
Publication Date :
January 2007
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This chapter talks about membrane adaptations, with emphasis on the adaptive changes occurring in prokaryotes; where relevant, the distinctive changes in eukaryotes will be compared. Genotypic adaptation refers to adaptive changes on an evolutionary time scale (usually longer than that for phenotypic adaptation), which involve an alteration in genetic structure, i.e., mutations occur and are positively selected if favorable to become established as part of the genome. Of particular relevance to the membrane adaptations of psychrophiles are the phenotypic and genotypic adaptations in lipid composition (the cellular “lipiome”), for which there is much information. Like the membranes of higher organisms, those of microorganisms are comprised mainly of proteins and lipids, together with a smaller amount of carbohydrate in the form of glycoproteins, glycolipids, or other molecules, organized as described originally in the Fluid-Mosaic Model of membrane structure. The presence of lipids that have a tendency to form non-bilayer phases gives a certain tension to the membrane and may be important in helping to drive processes such as sporulation and cell division that involve segregation of membranes. Microorganisms modify their membrane lipid fatty acyl composition in response to thermal changes by altering unsaturation, (methyl) branching, or chain length. The trans-unsaturated fatty acids are synthesized by direct and non-reversible isomerization of cisunsaturated fatty acyl chains without a saturated intermediate. The gene for the cis/trans isomerase enzyme has been cloned and the enzyme purified. Anteiso-branched fatty acids seem to be particularly associated with growth at low temperatures.
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Perception and Transduction of Low Temperature in Bacteria
- Authors: S. Shivaji, M. D. Kiran, S. Chintalapati
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Source: Physiology and Biochemistry of Extremophiles , pp 194-207
Publication Date :
January 2007
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This chapter focuses on the various mechanisms by which membrane fluidity is modulated in bacteria vis-à-vis its importance in cold adaptation. A detailed update on the perception and transduction of low-temperature signals in bacteria is also included. Subsequently, it was found that trans-monounsaturated are predominant in gram-negative bacteria and are synthesized by direct isomerization of cis-unsaturated fatty acids to trans-unsaturated fatty acids without shifting of a double bond. One of the predominant signal transduction mechanisms employed by bacteria is the phosphotransfer pathway commonly referred to as the two-component signal transduction system, which consists of a sensor kinase (histidine kinase) and a response regulator, found in bacteria, Archaea, and Eukarya. The first direct evidence for the two-component signal transduction mechanism involved in sensing cold has come from studies on Bacillus subtilis. Modulation in membrane fluidity appears to be crucial for low-temperature sensing in bacteria, and this is normally achieved by the conversion of saturated fatty acids to unsaturated fatty acids. Yet we are far from understanding many key aspects of bacterial signal transduction in response to low temperature.
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An Interplay between Metabolic and Physicochemical Constraints: Lessons from the Psychrophilic Prokaryote Genomes
- Author: Antoine Danchin
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Source: Physiology and Biochemistry of Extremophiles , pp 208-220
Publication Date :
January 2007
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This chapter summarizes the knowledge about the genomes of psychrophilic bacteria, a subclass of the cold-adapted bacteria, with emphasis on the specific selective features relevant to cold adaptation. A detailed analysis of the general features of genomes and proteomes from psychrophilic bacteria is presented. All investigators involved in sequencing the genomes of psychrophilic Bacteria looked for common features which would account for cold-adaptation. The genomes of psychrophilic bacteria also have the counterpart of major chaperonins such as the essential GroES GroEL complex. A remarkable observation poses interesting questions about the role of this complex. In the presence of molecular oxygen (dioxygen), this has the consequence that reactive oxygen species (ROS) are more frequent and stable for a longer time. Membrane fluidity can be increased in two ways: either by incorporating unsaturated fatty acids or by including branched-chain fatty acids in the diglycerides. Photobacterium profundum SS9 was found to exhibit enhanced proportions of both monounsaturated and polyunsaturated fatty acids when grown at a decreased temperature or elevated pressure. Three main features can be observed in the genomes and proteomes of these organisms: a variety of means to cope with ROS, a multiplicity of nucleic acid folding and unfolding devices, and, finally, a bias in the amino acid composition of their proteome.
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Meat, Poultry, and Seafood
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Source: Food Microbiology: Fundamentals and Frontiers, Third Edition , pp 105-140
Publication Date :
January 2007
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A muscle food, including meat, poultry, and seafood, is described as spoiled when it is considered unacceptable by consumers based on its sensory characteristics. Economic losses as well as the food wasted and loss of consumer confidence due to spoilage, however, are of major significance. Among bacteria, genera in the family Enterobacteriaceae, Photobacterium phosphoreum, Shewanella (Alteromonas) putrefaciens, Brochothrix thermosphacta, Pseudomonas spp., Aeromonas spp., and lactic acid bacteria have been found to be major contributors to muscle food spoilage, depending on the product type and the conditions surrounding the product. Chemical (chlorine, organic acids, inorganic phosphates, proteins, oxidizers, etc.) or physical (knife-trimming, cold or hot water, vacuum, and/or steam) agents and combinations of two or more agents simultaneously or in sequence are applied as carcass decontamination/sanitization treatments in the United States. Acidic decontamination of meat may lead to changes in the dominating microbial association and if combined with long-term storage may lead to development of yeasts. The microbial associations developing on muscle tissues stored aerobically at cold temperatures are characterized by an oxidative metabolism. The contribution of specific nutrients to bacterial behavior has been shown in single-culture or coculture studies. Glucose is the preferred energy substrate and the first to be used by various microorganisms growing on muscle foods. Glycolytic enzymes indigenous to muscle tissues participate in the postmortem glycolysis that ceases when the ultimate postmortem pH reaches 5.4 to 5.5. Freshness and safety of muscle food products are generally considered the most important contributors to quality.
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Chemical Preservatives and Natural Antimicrobial Compounds
- Authors: P. Michael Davidson, T. Matthew Taylor
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Source: Food Microbiology: Fundamentals and Frontiers, Third Edition , pp 713-745
Publication Date :
January 2007
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Antimicrobials are classified as traditional when they (i) have been used for many years, (ii) are approved by many countries for inclusion as antimicrobials in foods (e.g., lysozyme and lactoferrin, which are naturally occurring but regulatory-agency approved), or (iii) are produced by synthetic means (as opposed to natural extracts). Many organic acids are used as food additives, but not all have antimicrobial activity. Research suggests that the most active are acetic, lactic, propionic, sorbic, and benzoic acids. Acetic acid was the most effective antimicrobial in ground roasted beef slurries against Escherichia coli O157:H7 growth in comparison with citric or lactic acid. Sorbate is applied to foods by direct addition, dipping, spraying, dusting, or incorporation into packaging. The mechanism by which dimethyl dicarbonate (DMDC) acts is most likely related to inactivation of enzymes. A related compound, diethyl dicarbonate, reacts with imidazole groups, amines, or thiols of proteins. Lysozyme is most active against gram-positive bacteria, most likely because the peptidoglycan of the cell wall is more exposed. The primary use for sodium nitrite as an antimicrobial is to inhibit Clostridium botulinum growth and toxin production in cured meats. Sulfites may be used to inhibit acetic acid-producing bacteria, lactic acid bacteria, and spoilage bacteria in meat products. In the future, traditional food antimicrobials will continue to play an important role in food preservation.
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Nutrition and Media †
- Authors: David Emerson, Jane Tang
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Source: Methods for General and Molecular Microbiology, Third Edition , pp 200-214
Publication Date :
January 2007
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Recent intersections between geology and microbiology, as well as environmental engineering and microbiology, have provided even more creative approaches for developing selective media for novel organisms. This chapter places some emphasis on the nutritional needs of some of the broad groups of prokaryotes that have been discovered through using selective media as an enrichment tool. Other important aspects of nutrition that are considered in the chapter are requirements for vitamins, trace minerals, other growth factors, buffers to control pH, sterilization, and preparation of solid media. The essential nutritional requirements for prokaryotes can be divided into two classes: macronutrients and micronutrients. More than 30 elements are considered essential for cell growth; however, there are six nonmetals (carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus) and two metals (potassium and magnesium) that constitute 98% of the dry weight of prokaryotic organisms. Sodium is necessary for certain marine bacteria, phototrophs, and some anaerobes. Mixed hydrolysates are processed yeast or meat extracts to supplement the nutritional aspects of peptones by providing some amino acids, carbohydrates, nucleic acid fractions, organic acids, vitamins, and trace minerals lost during their manufacture. It is important to ensure that the blood and serum used in formulating bacteriological media are free of pathogens and potential health risks, especially in light of the recent bovine spongiform encephalopathy outbreaks.
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Tricarboxylic Acid Cycle and Glyoxylate Bypass
- Authors: John E. Cronan, Jr., and David Laporte
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Citation: Cronan, Jr. J, Laporte D. 2006. Tricarboxylic Acid Cycle and Glyoxylate Bypass, EcoSal Plus 2006; doi:10.1128/ecosalplus.3.5.2
- DOI 10.1128/ecosalplus.3.5.2
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The tricarboxylic acid (TCA) cycle plays two essential roles in metabolism. First, under aerobic conditions the cycle is responsible for the total oxidation of acetyl-CoA that is derived mainly from the pyruvate produced by glycolysis. Second, TCA cycle intermediates are required in the biosynthesis of several amino acids. Although the TCA cycle has long been considered a “housekeeping” pathway in Escherichia coli and Salmonella enterica, the pathway is highly regulated at the transcriptional level. Much of this control is exerted in response to respiratory conditions. The TCA cycle gene-protein relationship and mutant phenotypes have been well studied, although a few loose ends remain. The realization that a “shadow” TCA cycle exists that proceeds through methylcitrate has cleared up prior ambiguities. The glyoxylate bypass has long been known to be essential for growth on carbon sources such as acetate or fatty acids because this pathway allowsnet conversion of acetyl-CoA to metabolic intermediates. Strains lacking this pathway fail to grow on these carbon sources, since acetate carbon entering the TCA cycle is quantitatively lost as CO2 resulting in the lack of a means to replenish the dicarboxylic acids consumed in amino acid biosynthesis. The TCA cycle gene-protein relationship and mutant phenotypes have been well studied, although the identity of the small molecule ligand that modulates transcriptional control of the glyoxylate cycle genes by binding to the IclR repressor remains unknown. The activity of the cycle is also exerted at the enzyme level by the reversible phosphorylation of the TCA cycle enzyme isocitrate dehydrogenase catalyzed by a specific kinase/phosphatase to allow isocitratelyase to compete for isocitrate and cleave this intermediate to glyoxylate and succinate.
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Phase Variation of Streptococcus pneumoniae
- Author: Jeffrey N. Weiser
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Source: Gram-Positive Pathogens, Second Edition , pp 268-274
Publication Date :
January 2006
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Streptococcus pneumoniae undergoes spontaneous, reversible phenotypic variation, or phase variation, which is readily visualized as differences in colony morphology. This chapter describes phase variation in S. pneumoniae, the pneumococcus, and characterizes its relationship to colonization and the pathogenesis of infection. In particular, the focus is on the identification of variably expressed cell surface components as a means of gaining insight into the pathogenesis of pneumococcal disease at a molecular level. S. pneumoniae is highly proficient at colonization of its human host. The pneumococcus has the capacity to thrive in a number of diverse host environments, including the bloodstream and the mucosal surface of the nasopharynx. As is the case for other respiratory tract pathogens that frequently cause invasive infection, the ability of the pneumococcus to adapt to these varied environments requires changes in the expression of specific cell surface molecules.
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Two-Carbon Compounds and Fatty Acids as Carbon Sources
- Authors: David P. Clark, and John E. Cronan
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Citation: Clark D, Cronan J. 2005. Two-Carbon Compounds and Fatty Acids as Carbon Sources, EcoSal Plus 2005; doi:10.1128/ecosalplus.3.4.4
- DOI 10.1128/ecosalplus.3.4.4
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This review concerns the uptake and degradation of those molecules that are wholly or largely converted to acetyl-coenzyme A (CoA) in the first stage of metabolism in Escherichia coli and Salmonella enterica. These include acetate, acetoacetate, butyrate and longer fatty acids in wild type cells plus ethanol and some longer alcohols in certain mutant strains. Entering metabolism as acetyl-CoA has two important general consequences. First, generation of energy from acetyl-CoA requires operation of both the citric acid cycle and the respiratory chain to oxidize the NADH produced. Hence, acetyl-CoA serves as an energy source only during aerobic growth or during anaerobic respiration with such alternative electron acceptors as nitrate or trimethylamine oxide. In the absence of a suitable oxidant, acetyl-CoA is converted to a mixture of acetic acid and ethanol by the pathways of anaerobic fermentation. Catabolism of acetyl-CoA via the citric acid cycle releases both carbon atoms of the acetyl moiety as carbon dioxide and growth on these substrates as sole carbon source therefore requires the operation of the glyoxylate bypass to generate cell material. The pair of related two-carbon compounds, glycolate and glyoxylate are also discussed. However, despite having two carbons, these are metabolized via malate and glycerate, not via acetyl-CoA. In addition, mutants of E. coli capable of growth on ethylene glycol metabolize it via the glycolate pathway, rather than via acetyl- CoA. Propionate metabolism is also discussed because in many respects its pathway is analogous to that of acetate. The transcriptional regulation of these pathways is discussed in detail.
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Mupirocin
- Author: A. Bryskier
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Source: Antimicrobial Agents , pp 964-971
Publication Date :
January 2005
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Abstract:
Mupirocin is active against gram-positive cocci: it has moderate activity against gram-positive bacilli such as Listeria monocytogenes and Erysipelothrix rhusiopathiae; it is inactive against corynebacteria and Bacillus anthracis. It is inactive against Chlamydia spp. It is active against Staphylococcus aureus, whether the strains are susceptible or resistant to penicillin G, tetracyclines, erythromycin A, fusidic acid, lincomycin, chloramphenicol, or methicillin. Mupirocin has good activity against human and animal mycoplasmas. As mupirocin is highly serum protein bound, the in vitro activity is reduced in the presence of human serum. Mupirocin alters protein synthesis by blocking the incorporation of isoleucine into the peptide during synthesis. Since 1985, when mupirocin was introduced into clinical practice, two types of resistant strains have been described: those for which the MICs are between 8 and 256 µg/ml and those for which the MICs are 512 µg/ml. Extensive use of mupirocin has resulted in the emergence of resistant strains of S. aureus in the United Kingdom. Local application of mupirocin yields in situ concentrations of 20,000 mg/kg. Skin and skin structure infections are very common, but the use of topical antibiotics or other antibacterial agents is limited by a number of factors: the complexity of the anatomy of the skin, the bacterial ecology of the skin, and the nature of the microorganisms involved in these infections. Mupirocin eradicates methicillin-resistant strains of S. aureus colonizing the skin and produces an improvement in the healing of skin wounds.