Bacterial Structure and Function PDF

BACTERIAL STRUCTURE AND FUNCTION MOLECULAR STRUCTURE OF THE PLASMA MEMBRANE The blue spheres represent the head or the hydrophilic region attached to a hydrophobic tail. hydrophilic head consists of a glycerol molecule containing phosphate and a functional group (which could be sugar,...

BACTERIAL STRUCTURE AND FUNCTION MOLECULAR STRUCTURE OF THE PLASMA MEMBRANE The blue spheres represent the head or the hydrophilic region attached to a hydrophobic tail. hydrophilic head consists of a glycerol molecule containing phosphate and a functional group (which could be sugar, ethanolamine or choline) in this case ethanolamine which is bonded to the phosphate group. The fatty acid tail which makes the molecule hydrophobic is linked to the glycerol portion of the hydrophilic head via an ester linkage. The hydrophobic region of the lipid bilayer or unit membrane is formed when the fatty acid tails face each other in the interior, while the hydrophilic portion are the ones facing either the environment or the cytoplasm. Remember… Sterols strengthen the membranes of eukaryotic cells while some bacterial membranes are reinforced by weaker sterol-like molecules called hopanoids.  An ether linkage (in contrast to ester linkage in Bacteria and Eukarya) binds the glycerol to a hydrophobic side chain which is not a fatty acid. The hydrophobic portion of the membrane is made of a five- carbon hydrocarbon isoprene, instead of fatty acids. The membrane of an archaea is made of either phytanyl or biphytanyl. Isoprene serves as the parent structure of phytanyl and biphytanyl.  phytanyl component is a phosphoglycerol diether consists of C20 side chains called a phytanyl group while the biphytanyl component is a diphosphoglycerol tetraether with a C40 side chain. The lipid component of an Archaeal membrane may form a bilayer or a monolayer or a mix of both. To form a lipid monolayer, the ends of the inwardly pointing phytanyl groups of tetraether lipids are covalently linked at their termini. The membrane functions of an archaeal membrane are influenced by its chemistry. For example, the membranes of Crenarchaeota (major phylum of Archaea) contains crenarchaeol (biphytanyl), a common membrane lipid in their cells that has four C5 rings and one C6 ring. The presence of these rings affects the chemical properties of these lipids. Remember… Despite this unique archaeal membrane chemistry, the archaeal membrane still resembles the typical cytoplasmic membranes found in all cells, i.e., having an inner and outer hydrophilic structure and a hydrophobic interior. The polar head groups of their lipids can be sugars, ethanolamine or a variety of molecules but unlike Bacteria, hopanoids are not present in their cytoplasmic membrane.  MOLECULAR STRUCTURE OF THE BACTERIAL CELL WALL The cell wall of bacteria is capable of maintaining a concentration of dissolved solutes that can be as high as 2 atm (203 kPa). Similar to an automobile tire, this cell wall withstands pressure. It also protects the cell against osmotic lysis, and confers shape and rigidity on the cell. Destruction of the cell wall by antibiotics (penicillin and cephalosporin) make the bacteria susceptible to osmotic lysis. MOLECULAR STRUCTURE OF THE BACTERIAL CELL WALL The cell wall of bacteria is composed of a macromolecular network called peptidoglycan also known as murein. This molecule consists of a repeating disaccharide joined by polypeptides to form a lattice that surrounds and protects the entire cell. Peptidoglycan is found only the walls of Bacteria but absent in Archaea and Eukarya.  Overall structure of peptidoglycan The parallel polymers here are the disaccharides called glycan chains. The sugars are connected by peptides composed of four amino acids. The G monosaccharides in this illustration represents the N- acetylglucosamine (amino sugar derivative) or NAG while the M signifies the N-acetylmuramic acid (glucosamine plus a lactic acid group) or NAM.  Basic Components of a Peptidoglycan NAM, NAG and peptide cross links or a peptide interbridge between tetrapeptide chains The peptide cross links (consisting of 5 glycines) are formed between the tetrapeptide chains coming off from the NAMs.  Basic Components of a Peptidoglycan peptidoglycan monomer of S. aureus showing the pentapeptides coming off from NAM namely, L-alanine, D-glutamine, L-lysine, and two D- alanines. These amino sugars NAM and NAG are linked at the β-1,4 linkage. Gram-positive bacteria: thick peptidoglycan layer making up as much as 90% of the wall structure Many form several layers of stacked peptidoglycan but some form a single layer Associated with the peptidoglycan layer are teichoic acid, protein and lipoteichoic acid Teichoic acid - polymer of repeating ribitol units embedded in the cell wall Molecular structure of a ribitol teichoic acid glycerol phosphate or ribitol phosphate with attached molecules of glucose or D-alanine or both The phosphate group binds with individual alcohol molecules to form long strands which covalently links the molecule to the peptidoglycan Remember: Teichoic acids function to bind divalent metal ions such as Ca2+ and Mg2+ before they are transported into the cell. Teichoic acids that are covalently bonded to the membrane lipids rather than to the peptidoglycan are called lipoteichoic acids. Cell wall of a gram- negative bacteria: thick outer membrane small amount of peptidoglycan The outer membrane includes polysaccharides linked to lipids to form a complex structure known as the lipopolysaccharide layer or LPS porins and other membrane protein - associated with the outer membrane Cell wall of a gram- negative bacteria: thick outer membrane small amount of peptidoglycan The outer membrane includes polysaccharides linked to lipids to form a complex structure known as the lipopolysaccharide layer or LPS porins and other membrane protein - associated with the outer membrane  Solutes can enter and exit the outer membrane through the transmembrane protein porin. This protein has three identical polypeptides containing a channel in each subunit. Aggregation of the porin subunits form a hole about 1nm in diameter where very small molecules like nutrients can pass through.  Nonspecific porin allow very small hydrophilic substances to pass through its water-filled channels. Specific porins only allow the passage of substances that can bind to it, substances that are compatible to a specific porin binding site.  The periplasm is a 15nm space located between the outer surface of the cytoplasmic membrane and the inner surface of the outer membrane. It houses a variety of proteins including hydrolytic enzymes that degrade polymeric substances; binding proteins that initiate transport of substances; chemoreceptors responsible for chemotactic response and proteins that make extracellular structures like peptidoglycan and outer membrane from precursor molecules secreted through the cytoplasmic membrane. These proteins reach the periplasm through a protein exporting system located in the cytoplasmic membrane. Two components of lipopolysaccharide or LPS: O-specific (antigen) polysaccharide core polysaccharide The outer membrane does not function much on giving strength to the wall but rather serves as a barrier against lipophilic substances like antibiotics and other harmful chemicals that might enter and destroy the cytoplasmic membrane. Further dissection of the gram- negative cell wall shows that... The LPS replaces much of the phospholipid in the outer half of the outer membrane, and although the outer membrane is technically a lipid bilayer, its many unique components distinguish it from the cytoplasmic membrane. The outer membrane is anchored to the peptidoglycan layer by the murein lipoprotein or also known as Braun lipoprotein, a molecule that is found between the LPS layer and the peptidoglycan layer in the periplasm. Further dissection of the gram-negative cell wall shows that... The murein lipoprotein has an N-terminal cysteine (Cys) attached to three fatty acid side chains that are inserted in the inward facing leaflet of the outer membrane. The C-terminal lysine forms a peptide bond with the m-diaminopimelic acid of the peptidoglycan (murein). Components of the core polysaccharide (Salmonella) ketodeoxyoctonate (KDO) various seven-carbon sugars (heptoses) (Hep), glucose (Glu), galactose (Gal), rhamnose, and mannose one or more dideoxyhexoses such as abequose, colitose, paratose, or tyvelose. These sugars are connected in four- or five-membered sequences, which often are branched. When the sequences repeat, the long O-specific polysaccharide is formed. Among species, the O-specific polysaccharide is highly variable  N-acetylglucosamine (GluNac) and the lipid A fatty acids are linked through the amine groups of glucosamine (GlcN) The lipid A portion of LPS can be toxic to animals and comprises endotoxin complex  N-acetylglucosamine (GluNac) and the lipid A fatty acids are linked through the amine groups of glucosamine (GlcN). The lipid A portion of LPS can be toxic to animals and comprises endotoxin complex.  The lipid portion of the LPS called lipid A is not the typical glycerol lipid making up the cell membrane. Instead, the fatty acids are bonded through the amine groups from a disaccharide composed of glucosamine phosphate. The fatty acids commonly found in lipid A include caproic acid (C6), lauric (C12), myristic (C14), palmitic (C16), and stearic (C18) acids.  REACTION TO GRAM STAINING GRAM POSITIVE Crystal violet stains both gram + and gram- cells purple because the dye enters the cytoplasm of both types of cells When iodine (mordant) is applied, it forms large crystals with the dye (crystal- violet iodine) that are too large to escape through the cell wall. The application of alcohol dehydrates the peptidoglycan of gram positive cells to make it more impermeable to crystal- violet iodine.  REACTION TO GRAM STAINING GRAM NEGATIVE Alcohol dissolves the outer membrane of gram. negative cells and even leaves small holes in the thin peptidoglycan layer through which crystal violet-iodine diffuse. Because gram negative bacteria are colorless after alcohol wash, the addition of safranin (counterstain) turns the cells pink or red. Safranin provides a contrasting color to the primary stain (crystal violet) Although gram positive and gram negative cells both absorb safranin, the pink or red color safranin is masked by the darker purple dye previously absorbed by gram-positive cells.  Structure of a Mycobacterial Cell Envelope Members of the genus Mycobacterium like Mycobacterium tuberculosis (causative agent of tuberculosis) and Mycobacterium leprae (causative agent of leprosy) have a unique cell envelope because it exemplifies the characteristics of both gram-positive and gram-negative bacteria as well as features unique to a mycobacterium. Part of the core cell wall of a mycobacterium is the peptidoglycan linked chains of galactose called galactans. These galactans join with arabinans (polymer of five- carbon sugar arabinose) and form arabinogalactans. Arabinans in turn join with mycolic acids through an ester linkage. Mycolic acid has a hydroxy acid backbone with two hydrocarbon chains, one of them has a similar length with a typical membrane lipid (about 20 carbons), and the other chain is about three times longer. The long chain is composed of ketones, methoxyl groups, and cyclopropane rings. Mycolic acids can form a bilayer incorporated with sugar mycolates. It is like an outer membrane similar to that of a gram-negative bacterium. The mycolate layer also contains porin similar to that of a gram-negative bacterium.  Other protein inserted in the mycolate layer include virulence factor such as fibronectin-binding protein (Fbp) responsible for the ability of M. tuberculosis to invade macrophages. Phenolic glycolipids (phenol groups linked to sugar chains) are added to the outer ends of the sugar mycolates. The waxy surface and hydrophobicity of the phenol derivatives make them resistant to phagocytosis by macrophages.  Structure of the Cell Wall of an Archaebacteria The S layer is the paracrystalline surface layer made up of interlocking molecules of protein or glycoprotein. It can be hexagonal, tetragonal or in trimeric form depending upon the number and structure of the subunits composing it. strong enough to withstand osmotic pressure even without the other wall components However, when the other components are present, the S-layer is always the outermost wall and it is in direct contact with the environment.  CONJUGATION DNA is transferred from one bacteria to the other. Dependent upon thetra genes (encode the instructions for the bacterial cell to produce sex pilus) found in conjugative plasmids conjugative plasmids – fertility plasmids (F plasmid or F factor) of E. coli integrate into the bacterial genome  Conjugative transfer of plasmids with resistance genes has been an important cause of the spread of resistance to commonly used antibiotics within and between many bacterial species, since no recombination is required for expression in the recipient. Of all the mechanisms for gene transfer, this rapid and highly efficient movement of genetic information through bacterial populations is clearly of the highest clinical relevance.  Functions of Pili twitching motility of Pseudomonas aeruginosa and conjugation of bacteria using the sex pilus. https://drive.google.com/file/d/1c- JNBSvj9Qj_Wrr9xbaIXsuUCrnQ8Ye3/view?usp=drive_web&authuser= 0 https://drive.google.com/file/d/1SUgm_tlwJqLhc1RwA-- nKSRkE3Fmwwmz/view?usp=drive_web&authuser=0  Functions of Pili conjugation and cell movement. The sex or conjugative pili facilitate conjugation which is the genetic exchange between two cells. It also allows some bacteria particularly the pathogenic forms to adhere and invade host tissues. Aside from conjugation, type IV pili also support an unusual form of cell movement called twitching motility manifested by some forms of bacteria.  Functions of Pili Specifically, rod-shaped cells or the bacilli move by twitching and this movement is facilitated by the pili that are found only at the poles of the cells. These cells twitch along solid surfaces. When the cells twitch, their pili extend and then retract. These movements drag them through the solid surface. ATP is the source of energy needed for this twitching motility. Certain species of Pseudomonas and Moraxella move by twitching. Cell Inclusions found in the prokaryotic cell function as energy reserves and/or carbon reservoir cellular bodies enclosed by a thin membrane that serves as barrier between the materials it contained and the surrounding cytoplasm stored materials like carbon in inclusions reduces osmotic stress Carbon Storage Polymers Poly-β-hydroxybutyric acid (PHB) or polyhydroxyalkanoate (PHA) one of the most common inclusion bodies in prokaryotic organisms. lipids formed by β-hydroxybutyric acid units formed by the polymerization of β- hydroxybutyric acid (β-hydroxybutyrate) monomers connected by ester linkages then aggregate into granules hydroxybutyrate (C4) monomers vary in length from as short as 3 carbons (C3) to as long as 18 carbons (C18), and so, the more generic term Poly-β – hydroxyalkanoate (PHA) is often used to describe this class of carbon- and energy- storage polymers Carbon Storage Polymers excess of carbon leads to the synthesis of PHA then stored. Appropriate conditions lead to the breaking down of PHAs as carbon or energy sources many Bacteria produce PHAs, as do several extremely halophilic species of Archaea Glycogen polymer of glucose which is also a storage inclusion Similar to PHA, glycogen stores carbon and energy and is also formed when there is an excess carbon Polyphosphate, Sulfur, and Carbonate Minerals Inorganic phosphates (PO43-) - stored by many prokaryotic and eukaryotic microbes in the form of polyphosphate granules synthesized if there is an excess phosphate can be used as a source of phosphate when phosphate is limiting and it is needed for the biosynthesis of nucleic acids and phospholipids Illustrated here is the photomicrograph of the cells of Heliobacterium modesticaldum showing the dark polyphosphate granules. In some organisms, the synthesis of energy-rich compound ATP from ADP requires the breaking down of polyphosphates.  What you see here is a photomicrograph of the cells of the purple sulfur bacterium Isochromatium buderi. The periplasmic inclusions here are sulfur globules formed from the oxidation of hydrogen sulfide (H2S). Elemental sulfur (S0) is the product of the oxidation of hydrogen sulfide (H2S). In the process, electrons are generated to be used in the energy metabolism or carbon dioxide fixation. The produced S0 may accumulate in the cells in the form of microscopically visible granules and remain there as long as the source of reduced sulfur from which it was derived is still present. When the source of the reduced sulfur becomes limiting, the S0 in the granules is oxidized to sulfate  (SO42-) and eventually, the granules slowly disappear. Interestingly, although sulfur globules appear to reside in the cytoplasm, they are actually present in the periplasm. In these cells the periplasm expands outward to accommodate the growing globules as H2S is oxidized to S0 and then contracts inwards as S0 is oxidized to SO42-.