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Prokaryotic Form and Function Prokaryotic Profiles: The Bacteria and Archaea Chapter 3 • Prokaryotes can be distinguished from eukaryotes by: - the way their DNA is packaged (lack of nucleus and histones) - the makeup of their cell wall (peptidoglycan and other unique chemicals) - their internal structure (lack of membranebounded organelles) Copyright*©*The*McGraw3Hill*Companies,*Inc)*Permission*required*for*reproducAon*or*display.* The Structure of the Prokaryotic Cell The Structure of the Prokaryotic Cell • All bacterial cells possess: - a cell membrane • Some but not all bacterial cells possess: - flagella, pili, and fimbriae - cytoplasm - an outer membrane - ribosomes - plasmids - a cytoskeleton - inclusions - one (or a few) DNA chromosome(s) - endospores • Most bacterial cells possess: - a cell wall - a surface coating called a glycocalyx - intracellular membranes Prokaryotic Cell Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Prokaryotic Shapes and Arrangements In All Bacteria Cell (cytoplasmic) membrane—A thin sheet of lipid and protein that surrounds the cytoplasm and controls the flow of materials into and out of the cell pool. Bacterial chromosome or nucleoid—Composed of condensed DNA molecules. DNA directs all genetics and heredity of the cell and codes for all proteins. Ribosomes—Tiny particles composed of protein and RNA that are the sites of protein synthesis. In Some Bacteria Fimbriae—Fine, hairlike bristles extending from the cell surface that help in adhesion to other cells and surfaces. • Most prokaryotes exist as unicellular organisms - sometimes they can act as a group, in colonies or in biofilms Outer membrane—Extra membrane similar to cell membrane but also containing popolysaccharide. Controls flow of materials, and portions of it are toxic to mammals when released. Actin cytoskeleton—Long fibers of proteins that encircle the cell just inside the cell membrane and contribute to the shape of the cell. Cytoplasm—Water-based solution filling the entire cell. Cell wall—A semirigid casing that provides structural support and shape for the cell. • On average, prokaryotic cells are 1 µm (microns - can range from 0.05 -0.2 µm ( nanobes ) to 750 µm Pilus—An elongated, hollow appendage used in the transfer of DNA to other cells. Capsule (tan coating)—A coating or layer of molecules external to the cell wall. It serves protective, adhesive, and receptor functions. It may fit tightly or be very loose and diffuse. Also called slime layer and glycocalyx. Inclusion/Granule—Stored nutrients such as fat, phosphate, or glycogen deposited in dense crystals or particles that can be tapped into when needed. • Cells of one species may vary in shape and size - this is pleomorphism caused by variations in cell wall structure Plasmid—Double-stranded DNA circle containing extra genes. Flagellum—Specialized appendage attached to the cell by a basal body that holds a long, rotating filament. The movement pushes the cell forward and provides motility. In Some Bacteria (not shown) Endospore (not shown)— Dormant body formed within some bacteria that allows for their survival in adverse conditions. Intracellular membranes (not shown) © Science Photo Library RF/Getty Images Arrangement of Cocci Resulting from Different Planes of Cell Division Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Division in one plane Diplococcus Streptococcus (two cells) (variable number of coci in chains) Division in two perpendicular planes Tetrad (cocci in packets off our) Sarcina Division in (packet of several planes 8–64 cells) Irregular clusters (number of cells varies) Staphylococci and Micrococci` (a) (b) (c) External Structures Concept Check Which of the following structures are possessed by some but not all prokaryotes? Flagella – Prokaryotic Propellers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Bacterial locomotion A. Ribosomes B. A cell membrane C. One or more chromosomes D. Flagella • Three distinct parts Filament • Comprised of many proteins Hook Rod Rings (a) • 360o Flagellar Arrangements • • rotation Outer membrane Cellwall Basal body Cell membrane (b) Noreen R. Francis, Gina E. Sosinsky, Dennis Thomas and David J. DeRosier, Isolation, Characterization and Structure of Bacterial Flagellar Motors Containing the Switch Complex, Journal of Molecular Biology, Vol 235, Issue 4, 27 January 1994, Pages 1261–1270. Bacterial Movement Monotrichous: single flagellum • Bacteria move in response to chemical signals (chemotaxis) Lophotrichous: small bunches or tufts of flagella • Receptors bind extracellular molecules, which triggers flagellum to rotate Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Straight Tumble (a) General motility of a singular flagellum Straight (b) Peritrichous motility • Amphitrichous: flagella at both poles of the cell • Runs: smooth linear movement toward a stimulus • Peritrichous: flagella dispersed randomly over the surface of the cell • Tumbles: flagellar rotation reverses, causing the cell to stop and change its course Tumble Glycocalyx Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Composed of polysaccharides, proteins or both Prokaryotic Appendages • Varies in thickness • Fimbriae: used for attachment • Used to avoid phagocytosis and for adhesion (biofilms) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Pili: used for attachment and genetic exchange during conjugation Fimbriae Glycocalyx First colonists Organic surface coating Surface (a) Pili Cells stick to coating. Glycocalyx slime As cells divide, they form a dense mat bound together by sticky extracellular deposits. Additional microbes are attracted to developing film and create a mature community with complex function. Catheter surface © L. Caro/SPL/Photo Researchers Cell cluster (b) b: © Science VU–Charles W. Stratton/Visuals Unlimited Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capsule Cell Envelope • Bound more tightly to the cell, denser and thicker than a slime layer • Lies outside of the cytoplasm • Composed of two or three basic layers: - cell wall • Visible by negative staining • Produces a sticky (mucoid) character to colonies - cell membrane Capsule - outer membrane in some bacteria Cellbody • Encapsulated bacterial (a) cells generally have greater pathogenicity (b) (a): © John D. Cunningham/Visuals Unlimited; b: Fig 1 from Brett J. Pellock, Max Teplitski, Ryan P. Boinay, W. Dietz Bauer, and Graham C. Walker, A LuxR Homolog Controls Production of Symbiotically Active Extracellular Polysaccharide II by Sinorhizobium meliloti. Journal of Bacteriology, September15, 2002, Vol. 184, No. 18, p. 5067–5076. Reproduced with permission from American Society for Microbiology. Gram Positive Cell Peptidoglycan Cell Wall • Repeating framework of long glycan (sugar) chains crosslinked by short peptide (protein) fragments • Present in most bacteria • Thick peptidoglycan • Teichoic acid, lipoteichoic acid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Provides strength to resist rupturing due to osmotic pressure • One membrane Gram-Positive Wall teichoic acid Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (a) The peptidoglycan can be seen as a crisscross network pattern similar to a chainlink fence. CH2OH O Glycan chains M O O G O O M O G O O O G O O O O O O O G M Peptidoglycan Envelope CH2OH O G O M H3 C C H G O Cell membrane H3 C C H C O M G M M G O M M M G O M G M M G O M C M Membrane proteins G M Peptide cross-links (b) It contains alternating glycans (G and M) bound together in long strands. The G stands for N-acetyl glucosamine, and the M stands for N-acetyl muramic acid. Tetrapeptide O G O O O O G Lipoteichoic acid L –alanine L – alanine D – glutamate D –glutamate L –lysine D –alanine – glycine – glycine – glycine – D – alanine – glycine glycine L – lysine Interbridge (c) A detailed view of the links between the muramic acids. Tetrapeptide chains branching off the muramic acids connect by interbridges also composed of amino acids. It is this linkage that provides rigid yet flexible support to the cell and that may be targeted by drugs like penicillin. • Thin peptidoglycan Gram Negative Cell The Gram Stain: Developed by Hans Christian Gram (1884) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Lipopolysaccharide Gram-Negative Lipoproteins Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Porin proteins • Two membranes Lipopolysaccharides Outer membrane layer Phospholipids • Porins Step Microscopic Appearance of Cell Peptidoglycan Gram (+) Gram (–) Chemical Reaction in Cell Wall (very magnified view) Gram (+) Gram (–) 1. Crystal violet First, crystal violet is added to the cells in a smear. It stains them all the same purple color. Cell membrane Both cell walls affix the dye Membrane proteins 2. Gram s iodine Then, the mordant, Gram s iodine, is added. This is a stabilizer that causes the dye to form large complexes in the peptidoglycan meshwork of the cell wall. The thicker gram-positive cell walls are able to more firmly trap the large complexes than those of the gram-negative cells. Periplasmic space Membrane protein 3. Alcohol Application of alcohol dissolves lipids in the outer membrane and removes the dye from the peptidoglycan layer—only in the gram-negative cells. 4. Safranin (red dye) Because gram-negative bacteria are colorless after decolorization, their presence is demonstrated by applying the counterstain safranin in the final step. © McGraw-Hill Companies, Inc. Dye complex trapped in wall No effect of iodine Crystals remain in cell wall Outer membrane weakened; wall loses dye Red dye masked by violet Red dye stains the colorless cell Acid-Fast Bacteria • Mycobacterium sp. and Nocardia sp. • Contain mycolic acid (a wax) Cell-Wall Deficient Bacteria • Cell membrane stabilized by sterols, is resistant to lysis i.e. Mycoplasma pneumoniae • Other cell-wall deficient types called L forms are linked to infections Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Modified Gram-positive structure • Must use the acid-fast stain to ID important pathogens causing: - tuberculosis Mutation or chemical treatment GramPositive Cell wall (peptidoglycan) Cell membrane Cell membrane Peptidoglycan lost Protoplast (a) Mutation or chemical treatment - leprosy Outer membrane GramNegative (b) Outer membrane Peptidoglycan Cell membrane Cell membrane Peptidoglycan lost Spheroplast Cytoplasmic Membrane Lipopolysaccharide Case File Artwork • Located in the outermost layer of the outer membrane (OM) in gram-negative bacteria • Lipid A (endotoxin) stimulates fever and shock Case File Artwork • A lipid bilayer with proteins embedded - bacterial cell membranes contain primarily phospholipids (30%–40% of the membrane mass) and proteins (60%–70% of the membrane mass) • Provides a site for reactions - contains enzymes of respiration and ATP synthesis since prokaryotes lack mitochondria • A major action of the cell membrane is to regulate the passage of nutrients into and out of the cell The Cytoplasm Case File Artwork Bacterial DNA • 70-80% water Case File Artwork • Soluble proteins, salts, carbohydrates • DNA is aggregated in a dense area of the cell called the nucleoid • Site of nearly all chemical reactions • Many bacteria contain other, nonessential pieces of DNA called plasmids • Contains the DNA in the nucleoid Prokaryotic Ribosome Inclusions or Granules: Storage Bodies Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Two subunits (30S and 50S) • Total size is 70S (versus 80S in eukaryotes) • DNA of most bacteria exists in the form of a single circular bacterial chromosome Case File Artwork Large subunit (50S) Small subunit (30S) • Non-membrane bound granules • Usually for storage of nutrients The Cytoskeleton • 60% rRNA and 40% protein • Peptidoglycan layer determines shape of many bacteria • Translates mRNA into proteins • Others use protein fibers composed of actin and tubulin to alter cell shape Ribosome (70S) Bacterial Endospores A Typical Sporulation Cycle in Bacillus Species Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Dormant bodies Spore coats Chromosome Case File Artwork • Heat resistance due to calcium and dipicolinic acid content 1 Vegetative cell begins to be depleted of nutrients. Cortex Chromosome Core of spore Cell wall • Cortex, spore coats protect against radiation and chemicals • Metabolically active vegetative cells can undergo sporulation • Sporulation is not a reproductive function for most bacteria • When spores of Clostridium sp. are embedded in a wound with dead tissue, they can germinate, grow, and release toxins 9 8 2 Exosporium Spore coat Cortex Core Frees pore is released with the loss of the sporangium. 7 Cell membrane Germination: spore swells and releases vegetative cell. 3 Forespore Sporangium 4 Mature endospore Cortex 6 Chromosome is duplicated and separated. Cell is septated into a sporangium and forespore. Sporangium engulfs forespore for further development. Early spore 5 Cortex and outer coat layers are deposited. Sporangium begins to actively synthesize spore layers around forespore. © George Chapman/Visuals Unlimited Archaea • Prokaryotic microorganisms • Many are found in extreme environments (i.e. psychrophiles) • Different from members of the domains Bacteria and Eukarya in terms of: cell structure, metabolism, genetics Classification Systems Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Table 3.2 Comparison of Three Cellular Domains Characteristic Bacteria Archaea Eukarya Cell type Prokaryotic Prokaryotic Eukaryotic Chromosomes Single, or few circular Single, circular Several, linear Types of ribosomes 70S 80S Contains unique ribosomal RNA signature sequences Number of sequences shared with Eukarya Protein synthesis similar to Eukarya + 70S but structure is similar to 80S + 1 3 (all) + + Presence of peptidoglycan in cell wall + Cell membrane lipids Fatty acids with ester linkages Long-chain, branched Fatty acids with ester hydrocarbons with ether linkages linkages Sterols in membrane • For differentiating and identifying unknown microbial species i.e. Bergey s Manual of Determinative Bacteriology (phenotypic data) + • For studying prokaryotic relationships and origins i.e. Bergey s Manual of Systematic Bacteriology (rRNA sequencing data) (some exceptions) + Prokaryotic Classification Systems Case File Artwork • Bergey s Manual of Determinative Bacteriology organizes the prokaryotes into four major divisions based on cell wall structure: - Gracilicutes: gram-negative cell walls and thus are thin-skinned - Firmicutes: gram-positive cell walls that are thick and strong - Tenericutes: lack a cell wall and thus are soft - Mendosicutes: archaea with unusual cell walls Species and Subspecies in Prokaryotes Case File Artwork • Theoretically, a collection of bacterial cells, all of which share an overall similar pattern of traits and 70%–80% of their genes • Members of given species can show variations - subspecies, strain, or type are terms used to designate bacteria of the same species that have differing characteristics - serotype refers to representatives of a species that stimulate a distinct pattern of antibody (serum) responses in their hosts