Anatomy of Prokaryotic Cell 2023.ppt

Anatomy of Prokaryotic Cells Prokaryotic Cell Structure THE BACTERIAL CELL Cell Wall - Composed of peptidoglycan (polypeptide and dissacharide) The disaccharide makes up the backbone and is composed of N-acetyl glucosamine (NAG) and N- acetyl muramic acid (NAM) The polypeptide is found in ad...

Anatomy of Prokaryotic Cells Prokaryotic Cell Structure THE BACTERIAL CELL Cell Wall - Composed of peptidoglycan (polypeptide and dissacharide) The disaccharide makes up the backbone and is composed of N-acetyl glucosamine (NAG) and N- acetyl muramic acid (NAM) The polypeptide is found in adjacent rows and consist of tetra peptide chain (4 amino acids) attached to NAM. B-Lactam group Antibiotics interferes with in the bond formation between the disaccharide and the polypeptide making the cell wall weak and thus resulting in lysis of cells. • Peptidoglycan is a polysaccharide made of two glucose derivatives, Nacetylglucosamine (NAG) and N-acetylmuramic acid (NAM), alternated in long chains. • The chains are cross-linked to one another by a tetrapeptide that extends off the NAM sugar unit, allowing a lattice-like structure to form. • In many gram positive bacteria there is a cross-bridge of five amino acids such as glycine (peptide interbridge) that serves to connect one tetrapeptide to another. Gram Positive cell wall vs. Gram Negative Most bacteria may be classified into two types based on differences in their cell walls detected by the Gram staining procedure. The name refers to the Danish microbiologist Christian Gram, who developed the procedure to detect the presence of certain disease-causing bacteria. Gram-positive bacteria have a thick, peptidoglycan layer that retains a violet dye from the Gram stain procedure, causing the stained cells to appear purple under a microscope. Eg. Staphylococcus. In other groups of bacteria, the wall has thin peptidoglycan layer and does not retain the purple dye after Gram staining; such bacteria exhibit the background red dye and are characterized as Gram-negative. Ex. Escherichia coli. Their murein layer is covered on the outside with an additional outer membrane made up of lipids and Polysaccharides. This layer protects them from antibacterial enzyme, lysozyme, present in tears, saliva and other body fluids as well enable them to evade from certain antibiotics. Gram Positive cell wall Gram Positive Cell Wall (a) Thick peptidoglycan (b) Teichoic acid – composed of alcohol and phosphate. - Lipoteichoic acid- linked to the plasma membrane via a lipid anchor and span through the peptidoglycan layer - Wall teichoic acid- link to the peptidoglycan layer - Function of teichoic acid -regulate movement of positive ions in and out of the cell providing flexibility to the cell wall -Prevent extensive wall breakdown and possible lysis. - Make it possible to identify bacteria in the lab (c) No outer membrane (d) no pronounced periplasm (e) Less susceptible to breakage Gram Negative Cell Wall • Gram Negative Cell Wall (a) Thin peptidoglycan (b) outer membrane (lipopolysaccaride-LPS, lipophospholipids and lipoproteins) -Function: (i) promotes phagocytosis (ii) Barrier to certain antibiotics, digestive enzymes, detergents, bile salts and certain dyes) (iii)Permeable to nutrients - The peptidoglycan is bonded to lipoprotein in the outer membrane and in the periplasm (a jelly fluid between the outer membrane and the plasma membrane) (c) Well pronounced periplasm. Contains degradative enzymes and transport proteins) (d) No Teichoic acid (e) More susceptible to mechanical breakage Periplasm - Region between cytoplasmic membrane and the outer membrane (Gram negative cells) - Contains gel-like fluid - Filled with secreted proteins and enzymes - Enzymes such as B-lactamase present in the periplasm can neutralize the effect of antibiotics. Table: Correlation of the Gram stain with properties of bacterial cell walls Property Gram-positive Gram-negative Thickness of wall thick (20-80 nm) thin (10 nm) Number of layers 1 2-3 Peptidoglycan (murein) content >50% 10-20% Teichoic acids in wall present absent Protein/lipoprotein content 0-3% >50% Lipopolysaccharide content Sensitivity to penicillin 0 13 sensitive resistant Sensitivity to lysozyme sensitive resistant Functions/Uses of Cell Wall 1. Responsible for the shape of the cell. 2. Protection from changes in outside environment 3. Prevent bacterial cell from rupturing when water pressure inside the cell is greater than outside. 4. Serve as a point of anchorage for flagella 5. Contributes to the ability of some species to cause diseases. 6. Site of action of some antibiotics 7. Use to differentiate between types of bacteria ie. Gram positive and gram negative External Structure 1. Glycocalyx 2. Flagella 3. Axial filaments, 4. Pili (Pilus) and Fimbriae 1. Glycocalyx- substance secreted on the surface of prokaryotic cell wall. - viscous, sticky, gelatinous polymer that is found external to the cell and composed of polysaccharides, polypeptides or both - May be described as: (a)capsule- Thick, highly organized and firmly attached to the cell wall. (b)Slime layer-unorganized and only loosely attaches to the cell wall. Capsule in Acinetobacter species by gram negative staining Courtesy of Elliot Juni, Department of Microbiology and Immunology, The University of Michigan What is the importance of the capsule? 1. Contributes to bacterial virulence 2. Protect pathogenic bacteria from phagocytosis by the cell of the host. eg. Bacillus anthracis – the capsule has glutamic acid which prevents it from being destroyed by phagocytosis eg. Streptococcus pneumoniae- will cause pneumonia only if the cells are protected by polysaccharide capsule. It prevents phagocytosis and allow the bacterium to adhere to and colonize the respiratory tract. 3. The extra cellular polysaccharide enables a bacterium to survive by attaching to various surfaces in its natural environment. eg. Steptoccocus mutans causes dental caries. Allows bacteria to grow as a biofilm. 5. Helps prevent the loss of water and nutrients. Flagellum - Flagella       Flagella are long filamentous appendages that propel bacteria. Bacteria with no flagella are known as ATRICHOUS. Bacteria can also be grouped according to the number and distribution of flagella on their surface. In some bacteria, there is only a single flagellum – such cells are called monotrichous. In these circumstances, the flagellum is usually located at one end of the cell (polar). Some bacteria have a single flagellum at both ends – amphitrichous. However, many bacteria have numerous flagella; if these are located at both ends of the cell, this is described as lophotrichous. if they are distributed all over the cell, as peritrichous. The three basic part of a flagellum 1. Filament- long outer most region, constant in diameter and contain the globular protein flagellin arranged in several chains that intertwine and form a helix around a hallow core. 2. Hook- attached the filament to the base made of different protein than the filament. 3. Basal body – anchors the flagellum to the cell wall and plasma membrane. Made up of 2 or 4 rings that act as motor by using ATP and creates a rotation. How does the flagellum function? - Its causes movement by rotation from the basal body - As the flagella rotate, this motion pushes against the surrounding liquid and propels the bacterium. - Movement depends on continuous supply of energy - Continuous movement in 1 direction-run/swim but when it stops to change direction –tumble - Movement towards a stimulus is called TAXIS ie. Phototaxis or chemotaxis - Receptors on the walls of the bacteria determines attractant or repellant Chemotaxis B. Function of Flagella a) Movement of the cell in response to chemical signals i) Positive chemotaxis ii) Negative chemotaxis b) Chemotaxis is accomplished through a series of runs & tumbles 1. Runs and tumbles 2. Receptors for attractants and repellant Axial Filament (endo flagella) • Found in Spirochetes (long, slender helically coiled gram-negative bacteria) such as Treponema pallidium, that causes syphillis or Borrelia burgdorferi that’s causes lyme disease. • Two bundles of flagellin fibrils that arise at the ends of the cell beneath the outer membrane/sheath in periplasmic space and spiral around the cell (parallel but run in opposite direction). • The rotation of the filament produces a movement of the outer sheath that propels the spirochete in a spiral motion. Pili Pilus is longer than fimbriae but shorter than flagella. They are usually 1-2/cell. It is a hollow tubular structure made up of protein sub units called pilin, Their function is to facilitate conjugation, a primitive form of sexual act in prokaryotes. During conjugation plasmid can be transferred from one bacteria to another. Fimbriae FIMBRIAE- short appendages that aids in attachment to surfaces Location: poles of bacterial cells or evenly distributed over the entire surface. Mostly found in Gram negative bacteria Population –few to several hundreds Composition: protein called pilin arranged around a central core. Function: enable cell to adhere to surfaces. eg. Neisseria gonorrhoeae has fimbriae that allows it to attached and colonize mucous membranes Cell membrane (plasma membrane): Composition: Its structure is similar to eukaryotic cell membrane. It is made of lipid bilayer and embedded with protein. Prokaryotic cell membrane does not contain sterols but has a molecule called hopanoids. Hopanoids stabilize membrane structure. (a)Phospholipids bilayer - The phospholipid contain hydrophobic groups directed inward and hydrophilic groups directed to the exterior of the membrane. (b) Proteins - (i) peripheral proteins – lie at the inner or surface of the membrane -(ii) integral proteins- penetrate membrane completely Function: they function as enzymes that catalyses certain reactions Phospholipid • Plasma membrane Function- is semi-permeable & found beneath the cell wall. 1. Selective barrier through which materials enter and exit the cell. (protein is too large but water, oxygen, carbon dioxide and glucose can pass) 2. Break down of nutrients and production of energy. Membrane contains enzymes that break down nutrients and produce ATP through Electron Transport Chain. 3, Involved in cell to cell interaction. Toxins causing cholera and botulism enter their target cells by binding to the glyco-protein. 4. Photosynthetic bacteria contain sac-like, tubular or sheet-like infoldings containing photosynthetic pigment, bacteriochlorophyll. 5. PM is associated with motility, cell division and endospore formation. Figure 4.18: The principle of osmosis - Overview. Glass tube Rubber stopper Rubber band Sucrose molecule Cellophane sack Water molecule (a) At beginning of osmotic pressure experiment Cytoplasm Solute (b) At equilibrium Plasma membrane Cell wall Water (c) Isotonic solution — no net movement of water Microbiology: An Introduction, 9e by Tortora, Funke, Case (d) Hypotonic solution — water moves into the cell and may cause the cell to burst if the wall is weak or damaged (osmotic lysis) (e) Hypertonic solution — water moves out of the cell, causing its cytoplasm to shrink (plasmolysis) Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings. Atypical cell wall Genus Mycoplasma: Mycoplasmas are the smallest and simplest self-replicating bacteria. Unlike all other prokaryotes, the mycoplasmas have no cell walls, and they are consequently placed in a separate class Mollicutes. They were once believed to be virus because they could pass through bacterial filters. Their plasma membrane have lipids called STEROLS which help protect them from lysis (rupture). (A). Differences in the cell envelope of Gram-positive, acid-fast and Gram-negative bacteria. AG, arabinogalactan; GL, glycolipid; LAM, lipoarabinomannan; LP, lipoprotein; LPS, lipopolysaccharide; LTA, lipoteichoic acid; MA, mycolic acid; MAP, membrane-associated protein; OM, outer membrane; PG, peptidoglycan; PM, plasma membrane; TA, teichoic acid . (B), PG structure revealing the unique features present in mycobacterial cells highlighted in blue. Cell Wall of Archaea - Many lack wall or have unusual wall composed of a protein or glycoprotein S-layer, but not peptidoglycan. Some contain pseudo-peptidoglycan (a modified form of peptidoglycan. • Their are four fundamental differences between the archaeal cell membrane and those of all other cells: (1) chirality of glycerol, (2) ether linkage, (3) isoprene chains, and (4) branching of side chains. BACTERIAL CYTOPLASM (CYTOSOL) • • • • – 80% weight water – Containing precursors, enzymes, ribosomes, inclusions, nucleoid and plasmids. consist of metabolites and nutrients in solution. The major constituent of the cell pool is water, which acts a solvent for carbohydrates, salts, amino acids and enzymes. nucleoid- consist of double stranded DNA plasmids- smaller pieces of DNA which codes for unique charcteristics that aid the cell in some specific circumstances not vital for survival. Ribosomes- protein synthesis Chromosomal DNA Found within a central location known as nucleoid – Stores the genetic information • • • • 4.7 x 106 bp 3 x 109 Da in molecular weight Containing 2,000 - 4,000 genes Interacts w/ membrane during replication • Single, circular, double stranded DNA • Consists of all DNA required by cell for metabolism and survival. Chromosomal DNA • PLASMIDS – Small extrachromosomal circular dsDNA molecules – Not crucial for bacterial survival – Confers bacteria w/ selective advantage • Synthesis of toxins - pathogenicity • Antibiotic resistance - Resistance-transfer factors RTFs • Synthesis of the F pilus • Metal tolerance • Novel enzymatic activities • Synthesis of Adhesins – Utilized as cloning vehicles in Biotechnology Inclusions Membraneless deposits - Accumulate certain nutrients (a) Volutin granules - inorganic phosphates (b) Polysaccharides granules- stores glycogen and starch (c) Lipid inclusions- stores lipid (d) Sulfur granules- stores sulphur compounds (e) Carboxysomes- contain enzymes ribulose 1,5 diphosphate carboxylase eg. Nitrifying bacteria (f) Magnetosomes- inclusions of Iron Oxide by gram negative bacteria. They act as magnets, use to move towards a suitable attachment site. Cytoskeleton: There exists some protein fibers that act as cytoskeleton. The cytoskeletal structures play important roles in cell division, cell polarity, cell shape regulation, plasmid partition, and other functions.. CYTOPLASMIC INCLUSIONS Bacterial Ribosomes • Site of protein synthesis • Relative size and density of ribosomes and their subunits expressed as distinct unit (S) • Two units of prokaryotic ribosomes: 50S + 30S= 70S • Eukaryotic ribosomes: 80S (60s + 40s) Endospores - Are resting structures formed by some bacteria Function: they allow survival during adverse environmental conditions. Structure: highly durable dehydrated cells with thick walls and additional layers. - They can survive extreme heat, lack of water, and exposure to many toxic chemicals and radiation. Formation: process is known as sporulation or sporogenesis – Developmental process leading to survival of the species under harsh conditions for growth – Genera that produce: Bacillus, Clostridium – Resistance to all disinfection, chemicals, radiation, freezing, heat (Calcium dipicolinate) – Dehydrated & metabolically dormant structure