Lecture 02 v2.pdf

BIOL371: Microbiology Lecture 2 – Microbial cell structure and function Topics of today 1. The cell envelope 2. Cell surface structures 3. Cell locomotion 4. Taxis Materials covered:  Chapter 2.1, 2.3-2.7, 2.8 (excluding sporulation cycle), 2.9 (only section on flagella and flagellation), 2.11,...

BIOL371: Microbiology Lecture 2 – Microbial cell structure and function Topics of today 1. The cell envelope 2. Cell surface structures 3. Cell locomotion 4. Taxis Materials covered:  Chapter 2.1, 2.3-2.7, 2.8 (excluding sporulation cycle), 2.9 (only section on flagella and flagellation), 2.11, 2.12  Figures 2.1-2.4, 2.7-2.21, 2.23-2.26, 2.28, 2.30, 2.39-2.40 The cell envelope of prokaryotic microbes  Series of layered structures surrounding cytoplasm and governing interactions with environment  Cytoplasmic membrane – surrounds the cytoplasm and separates it from the environment  Cell wall – confers structural strength to cells  Outer membrane – Gram-negative bacteria  S-layers – found in many bacteria and almost all archaea Phospholipid bilayer membrane  Bacterial and eukaryotic cytoplasmic membranes have the same organization  Phospholipid bilayer containing embedded proteins  Hydrophobic (water-repelling) region made up of “tails” of fatty acids  Hydrophilic (water-attracting) exposed to the environment and the cytoplasm – made up of “head” group of fatty acid which is linked to glycerol phosphate, which in turn is bonded with one of several functional groups (such as sugars, ethanolamine, or choline) Membrane proteins  Integral membrane proteins – embedded in the cytoplasmic membrane  Transmembrane proteins – integral membrane proteins that extend across the bilayer  Peripheral membrane proteins – loosely attached to the membrane Archaeal cytoplasmic membrane  Ether linkage in phospholipids in contrast to bacteria and eukarya that have ester linkages in phospholipids  Archaeal lipids have isoprenes instead of fatty acids The chemical bond The R-group Bacteria Fatty acid (CH2)n-CH3 Isoprene Archaea Functions of cytoplasmic membrane  Permeability barrier  Polar and charged molecules must be transported.  Transport proteins accumulate solutes against the concentration gradient.  Protein anchor  Holds proteins in place.  Energy conservation and consumption  Generation of proton motive force. Bacterial cell wall  Need to withstand osmotic/turgor pressure to prevent cell lysis – 2 atm or ~automobile tire pressure caused by high concentration of solutes  Maintains cell shape and rigidity  Based on a staining procedure developed by Han Christian Gram in 1884, Bacteria are broadly classified into two groups: Grampositive and Gram-negative based on Gram stain (organization and cell wall structures)  Gram staining is based on differential staining with a crystal violet-iodine complex and a safranin counterstain Organization differences of the cell envelope between Gram-positive and Gram-negative bacteria  Gram-positive cell envelope  Cytoplasmic membrane + thick cell wall  Gram-negative cell envelope  Cytoplasmic membrane, thin cell wall, outer membrane, periplasm (between cytoplasmic and outer membranes)  Gram stain reaction determined by cell wall thickness does not always correlate with envelope structure. Cell envelopes of bacteria Gram-positive Transmission electron micrographs Gram-negative Bacterial cell wall made up of peptidoglycan     Peptidoglycan: rigid polysaccharide layer that provides strength Found in all Bacteria with a cell wall Not found in Archaea or Eukarya Glycan tetrapeptide contains  Sugar backbone (alternating modified glucose (N-acetylglucosamine and N-acetylmuramic acid) joined by β-1,4 linkages  Short peptide attached to N-acetylmuramic acid  Amino acids vary between species  L-alanine, D-alanine, D-glutamic acid, and either L-lysine or diaminopimelic acid (DAP) Peptidoglycan: repeating unit of glycan tetrapeptide Beta-1,4 bond between N-acetylmuramic acid an N-acetylglucosamine is sensitive to the hydrolysis of the enzyme lysozyme (found in human secretions such as saliva, milk; major defense against bacterial infection) Peptidoglycan structure  Peptidoglycan strands run parallel around cell circumference  Cross-linked by covalent peptide bonds  Gram-negative crosslinks between DAP amino and D-alanine carboxyl on adjacent glycan strands  Primarily single layer  Gram-positive cell wall is 15 or more layers thick  Stabilized by horizontal and vertical cross-links often containing interbridges (e.g., five glycine residues in Staphylococcus aureus) Gram-positive bacterial cell wall  Commonly have teichoic acids (acidic molecules) embedded in cell wall and covalently linked to peptidoglycan  Lipoteichoic acids covalently bound to membrane lipids  The antibiotic penicillin block peptide crosslinks Archaeal cell envelope  Cytoplasmic membrane structure differs from bacteria  Lack peptidoglycan  Typically lack outer membrane  Most lack polysaccharide wall, instead have S-layer (rigid protein shell to prevent cell lysis) Some archaea have pseudomurein cell wall  Methanogens have polysaccharide (pseudomurein) cell wall  Similar to peptidoglycan  Alternating N-acetylglucoamine and Nacetyltalosaminuronic acid (replaces Nacetylmuramic acid of peptidoglycan)  Beta-1,3 glycosidic bonds instead of β1,4 bonds in peptidoglycan  Amino acids are all L-steroisomer  Cannot be hydrolyzed by lysosyme Overview of the Gram-negative bacterial cell envelope  Most Gram-negative bacteria have an outer membrane on the surface  Lipid bilayer external to cell wall  Contains polysaccharides covalently bound to lipids; hence called lipopolysaccharide layer (LPS)  Facilitates surface recognition and adds strength Structure and activity of the lipopolysaccharide layer  Polysaccharide made up of core polysaccharide and O-polysaccharide (composition highly variable and highly antigenic, which provides the basis for serotyping)  Lipid A portion – lipid A is an endotoxin responsible for toxicity of pathogenic Gramnegative bacteria causing gas, diarrhea and vomiting  Braun lipoprotein anchors outer membrane to the peptidoglycan Structure of bacterial lipopolysaccharide  Chemical structure of lipid A and polysaccharide vary among Gram-negative bacteria  Core polysaccharide contains ketodeoxyoctonate (KDO), heptoses (Hap), glucose (Glu), and galactose (Gal)attached to lipid A through KDO  O-specific polysaccharide made up hexoses and modified deoxysugars in a chain of up to 30 repeating units (each unit – three sugars in the backbone and one side sugar) Periplasm and porins of the outer membrane  Periplasmic space is located between cytoplasmic and outer membranes of ~15 nm in width  Home to many hydrolytic enzymes, sensing proteins, and transport proteins  Porins are channel proteins for the transport of small hydrophilic molecules  Can be non-specific water-filled pores or specific for certain small molecules  Not permeable to large molecules and proteins S-layers  Found in many bacteria and almost all archaea  Paracrystalline structure consisting of proteins and glycoproproteins Cell envelope of Caulobacter crescentus  If present, always outermost layer  Functions include strength, protection from lysis, conferring shape, creating periplasmic-like space, facilitating cell surface interactions, promoting adhesion, protecting cell from host defenses S-layer fragment from Aquaspirillum Examples of alternative configurations of the cell envelope  Outer S-layer surrounding Gram-positive or Gram-negative bacterium  Many archaea only have S-layer outside cytoplasmic membrane  Archaea with outer membrane  Pseudomurein cell walls of archaea with or without S-layer Vibrio cholera, typical Gram-negative  Some bacteria and archaea lack cell walls, have tough cytoplasmic membrane (e.g., sterols)  Mycoplasma (bacteria)  Thermoplasma (archaea) Mycoplasma pneumoniae, only cytoplasmic membrane Bacterial capsules and slime layers  Sticky polysaccharide coat outside cell envelope  Capsule: tight matrix tightly attached to cell envelope  Slime layer: loosely attached and easily deformed  Functions:  Assist in attachment to surface  Role in development and maintenance of biofilms  Contribute to infectivity  Prevent dehydration/desiccation Cell surface structures of bacteria – pili and fimbriae  Pili: thin filamentous protein structures of ~2-10 nm wide  Produced by all Gram-negative and many Gram-positive bacteria  Conjugative/sex pili facilitate genetic exchange between cells (conjugation)  Enable cells to stick to cell surface or biofilms  Fimbriae: short pili mediating attachment Unique attachment structures of archaea: hami  Hamus/hami of SM1 group archaea: similar to pili except for barbed termini  Assist in surface attachment, forming biofilms Cell inclusions to store carbon polymers and minerals Inclusion bodies:  Enclosed by thin protein membrane to reduce osmotic stress  For storage and special functions  Carbon storage polymers  Synthesizes when carbon in excess  Broken down as carbon or energy when needed  Poly-β-hydrobutyrate (PHB): use in production of biodegradable plastics  Glycogen: glucose polymers  Storage of polyphosphate, sulfur and other minerals Poly-β-hydrobutyrate Floating cells: gas vesicles Cyanobacteria and some other bacteria and archaea produce gas vesicles made of protein to control buoyancy in the water column Clusters of gas vesicles Phototrophic organisms benefit from this as they can position themselves at depths of optimal light availability Magnetotactic bacteria and magnetosomes Magnetosome  Acts as an internal magnet to orient within magnetic field  Uses the Earth’s magnetic field to move  Allow cell to undergo magnetotaxis: migration along magnetic field lines Membrane (arrow) surrounding each magnetosome in a chain Magnetospirillum magnetotacticum Endospores – survival structures  Specialized spores – survival structures to endure unfavorable growth conditions  Highly differentiated, dormant cells resistant to heat, radiation, chemical exposure, drying, lack of nutrient  Ideal for dispersal via wind, water, or animal gut  Found only in some Gram-positive bacteria; e.g., Bacillales and Clostridiales Clostridium pascui Cell motility  Many microbes are motile. Motility mechanisms includes:  Bacteria use of flagella for propulsion  Gliding along surfaces, propelled by secretion of polymers or use of glide proteins  Microbes use motility to find food or run away from hostile environments Single polar flagellum Peritrichous flagella Lophotrichous flagella Flagella and flagellation  Flagella – long, thin appendages of 15-20 nm wide (archaella of archaea are short filaments similar in composition as bacterial pili)  Different arrangements: polar, amphitrichous, perichitricous, lophotricous  Variable rotational speed relative to proton motive force (a) Peritrichous (b) polar Taxis  Taxis: directed movement of bacteria and archaea in response to stimuli  Directed movement enhances access to nutrients or allows avoidance of damage/death  Chemotaxis: response to chemicals  Phototaxis: response to light  Magnetotaxis: movement along magnetic field lines  Aerotaxis: movement towards an optimal oxygen level  Osmotaxis: movement towards or away from environments of high ionic strength  Hydrotaxis: movement towards water Chemotaxis in E. coli Sense attractants and repellents with chemoreceptors  Absence of chemical gradient, cells move in a random fashion  Run – counter-clockwise rotation of flagella  Tumble – clockwise rotation of flagella  Presence of chemical gradient  Runs become longer and tumbles less frequently  Cells compares its chemical environment to that of a few second before Measuring chemotaxis  Insert a capillary tube containing an attractant or a repellent in a suspension of motile bacteria  Chemical concentration decreases with distance from tip  Chemotactic bacteria swarm toward attractant, increasing number of cells in capillary  Screen nutrients for their preference by bacteria