Bacterial Cell Structure and Function PDF

2 Microbial Cell Structure and Function 2.1 Cell Morphology Morphology is __________ & __________ of an organism. Common morphologies of prokaryotic cells: a) Spherical or ovoid b) Cylindrical c) Curved or spira...

2 Microbial Cell Structure and Function 2.1 Cell Morphology Morphology is __________ & __________ of an organism. Common morphologies of prokaryotic cells: a) Spherical or ovoid b) Cylindrical c) Curved or spiral After cell division: 2.1 Cell Morphology a) Cocci (coccus singular) b) Rods c) Spirillum (spirilla) c) Spirochete After cell division: Figure 2.1 Secondary groupings Size: Lower limit 700 µm in diameter Size range for eukaryotic cells: 2 to >600 µm in diameter May be selective forces involved in setting the morphology Advantage of being small= Optimization for nutrient uptake/ waste elimination. Typically, faster growth. Figure 2.3 Surface area and volume relationships in cells. 2.3 The Cytoplasmic Membrane Main function: selective permeability 8-10nm wide Has embedded proteins Hydro________ = fatty acids Hydro_________= glycerol + phosphate and another functional group (e.g., sugars, ethanolamine, choline) Figure 2.5 2.3 The Cytoplasmic Membrane Comparisons: Linkages in phospholipids (ether) of Archaea vs. linkages in phospholipids (ester) Bacteria & Eukarya Archaea isoprenes instead of fatty acids. Take home → differences exist Cytoplasmic membrane function 1. permeability barrier Polar and charged molecules must be transported. Transport proteins accumulate solutes against the concentration gradient. 2. protein anchor holds transport proteins in place 3. energy conservation and consumption generation of proton motive force Cell Walls: Peptidoglycan (in bacteria) Gram stain- some cells appear pink and others purple (2 groups) Why? → cell wall structure differences gram-negative cell wall at least two layers: lipopolysaccharide (LPS) and peptidoglycan gram-positive cell wall 90% peptidoglycan common to have teichoic acids covalently bound thicker, primarily one layer of peptidoglycan  2.4 Bacterial Cell Walls: Peptidoglycan rigid layer that provides strength cross-linked differently in gram-negative bacteria and gram-positive bacteria can be destroyed by lysozyme (enzymes that cleaves bond between sugars) found in human secretions, major defense against bacterial infection  Peptidoglycan Synthesis Pre-existing peptidoglycan needs to be temporarily severed to allow newly synthesized peptidoglycan to form. Must export through cytoplasmic membrane. Target for antibiotics: More prominent in Gram+ bacteria Transpeptidation inhibited by the antibiotic penicillin Continued activity of autolysins weakens peptidoglycan, and cell bursts. As always, exceptions… Prokaryotes that lack cell walls Mycoplasmas group of pathogenic bacteria related to gram-positives Thermoplasma Archaea 2.5 LPS: The Outer Membrane Most of cell wall composed of outer membrane or the lipopolysaccharide (LPS) layer. Small amount of total cell wall contains peptidoglycan. barrier against antibiotics and other harmful agents Braun lipoprotein- spans LPS and peptidoglycan layer LPS consists of core polysaccharide, O-polysaccharide, and lipid A. LPS replaces most of phospholipids in outer half of outer membrane. endotoxin: lipid A, the toxic component of LPS MiniQuiz 2.5 #1 Describe the major chemical components in the outer membrane of a gram-negative bacteria. Cell Walls (Archaea) No peptidoglycan Most common cell wall type- S-layers, paracrystalline structure Pseudomurein- Polysaccharide similar to peptidoglycan Found in cell walls of certain methanogenic Archaea cannot be destroyed by lysozyme and penicillin 2.7 Cell Surface Structures Capsules and slime layers not considered part of cell wall (no significant structural strength) polysaccharide layers may be thick or thin, rigid or flexible Tightly attached → “capsule” (stains with India ink) Loosely adhered → “slime layer” 2.7 Cell Surface Structures Function: assist in attachment to surfaces role in development and maintenance of biofilms virulence factors: protect against phagocytosis prevent dehydration/desiccation 2.7 Cell Surface Structures Fimbriae and pili Filamentous protein structures Fimbriae enable organisms to stick to surfaces or form pellicles (thin sheets of cells on a liquid surface). Pili are typically longer, and fewer (1 or a few) Conjugative/sex pili facilitate genetic exchange between cells (conjugation). Type IV pili adhere to host tissues and support twitching motility (e.g., Pseudomonas and Moraxella). 2.7 Cell Surface Structures Hamus/hami Archaeal “grappling hooks” assist in surface attachment, forming biofilms. structurally resemble type IV pili except for barbed terminus, which attaches cells to surfaces or each other 2.8 Cell Inclusions Inclusions function as energy reserves, carbon reservoirs, and/or have special functions. Enclosed by thin membrane Reduces osmotic stress Carbon storage polymers 2.8 Cell Inclusions Storage: Polyphosphate granules: inorganic phosphate Sulfur globules: sulfur found in periplasm, oxidized to sulfate (SO42–) Carbonate minerals: biomineralization of barium, strontium, and magnesium Magnetosomes: magnetic iron oxides; allow cell to undergo magnetotaxis: migration along magnetic field lines 2.9 Gas Vesicles Buoyancy Conical-shaped, gas-filled structures made of protein Impermeable to water and solutes Molecular structure Gas vesicles are composed of two proteins function by decreasing cell density, increasing buoyancy Figure 2.25 Buoyant cyanobacteria and their gas vesicles.  2.10 Endospores Formed during endosporulation or sporulation Highly differentiated cells resistant to heat, harsh chemicals, and radiation Survival structures to endure unfavorable growth conditions “Dormant” stage of bacterial life cycle Ideal for dispersal via wind, water, or animal gut Present only in some gram-positive bacteria (e.g., Bacillus and Clostridium) 2.10 Endospores Formation and germination vegetative cell converted to non-growing, heat-resistant, light- refractive structure (Figure 2.28) only occurs when growth ceases due to lack of essential nutrient such as carbon or nitrogen 2.10 Endospores Formation and germination contains dipicolinic acid, enriched in Ca2+ many layers: exosporium (outermost), spore coats, cortex, core can remain dormant for years but converts rapidly back to being vegetative three steps: activation, germination, and outgrowth (Figure 2.29) activation: heated for several minutes at elevated but sublethal temperature germination: rapid (minutes), loss of refractility and loss of resistance to heat and chemicals outgrowth: swelling from water uptake and synthesis of RNA, proteins, and DNA 2.11 Flagella, Archaella, and Swimming Motility Flagella/archaella: structure that assists in swimming in Bacteria and Archaea, respectively tiny rotating machines Flagellar and flagellation long, thin appendages different arrangements: polar, lophotrichous, amphitrichous, peritrichous increase or decrease rotational speed 2.11 Flagella, Archaella, and Swimming Motility Flagellar structure and activity helical in shape consists of several components Filament composed of flagellin. reversible rotating machine 2.11 Flagella, Archaella, and Swimming Motility Archaella half the diameter of bacterial flagella (10–13 nm) move by rotation composed of several different filament proteins with little homology to bacterial flagellin speeds vary from 0.1–10x Escherichia coli structurally similar to type IV pili 2.12 Gliding Motility Bacteria only; no Archaea Slower and smoother than swimming Requires surface contact (Figure 2.40) Mechanisms excretion of polysaccharide slime type IV pili/twitching motility gliding-specific proteins (adhesion complexes or other specialized proteins) (Figure 2.41) 2.13 Chemotaxis and Other Taxes Taxis: directed movement in response to chemical or physical gradients chemotaxis: phototaxis: aerotaxis: osmotaxis: hydrotaxis: 2.13 Chemotaxis and Other Taxes Chemotaxis best studied in E. coli (peritrichous) “run and tumble” behavior run: smooth forward motion, flagellar motor rotates counterclockwise tumble: stops and jiggles, flagellar motor rotates clockwise, flagellar bundle comes apart move by rotation monitor/sample environment with chemoreceptors that sense attractants and repellents 2.13 Chemotaxis and Other Taxes Phototaxis using photoreceptors allows phototrophic organisms to optimize position for light harvest. (Figure 2.44a) Scotophobotaxis: Entering darkness causes cell to tumble, reverse direction, head back to light. aerotaxis (Figure 2.43f), osmotaxis, hydrotaxis Poll Questions, Quizlet, or Kahoot Kahoot: Lecture 2 Think it through: Bacterial cells can sense a gradient ONLY by identifying ________ changes rather than ________ differences because of their small ________. A. spatial / temporal / cell size B. spatial / temporal / chemoreceptors C. temporal / spatial / cell size D. temporal / spatial / chemoreceptors 2.14 The Nucleus and Cell Division Eukaryotes contain a membrane-enclosed nucleus Other organelles include mitochondria, Golgi complex, lysosomes, endoplasmic reticula, microtubules, and microfilaments. (Figure 2.45) Some have motility (flagella or cilia). Some have cell walls. Membranes contain sterols that lend structural strength. 2.14 The Nucleus and Cell Division Nucleus: contains the chromosomes (Figure 2.46) DNA is wound around histones. enclosed by two membranes that interact with nucleoplasm (inner membrane) and cytoplasm (outer membrane) Within the nucleus is the nucleolus. site of ribosomal RNA synthesis 2.14 The Nucleus and Cell Division Cell division mitosis (Figure 2.47) normal form of nuclear division in eukaryotic cells results in two diploid (two copies of each chromosome) daughter cells meiosis specialized form of nuclear division results in four haploid (one copy of each chromosome) gametes 2.15 Mitochondria, Hydrogenosomes, and Chloroplasts All specialize in energy metabolism. Mitochondria (Figure 2.48) respiration and oxidative phosphorylation for aerobic eukaryotes few—1000+ per cell surrounded by two membranes cristae: folded internal membranes contain enzymes needed for respiration and ATP production matrix: innermost area of mitochondrion contains citric acid enzymes 2.15 Hydrogenosomes found in anaerobic, strict fermenters (e.g., Trichomonas and some protists) similar size to mitochondria lack TCA cycle enzymes and cristae Major function is oxidation of pyruvate to H2, CO2, and acetate. Some methanogenic Archaea live in some anaerobic eukaryotes and consume H2 and CO2, producing CH4; acetate is secreted. 2.15 Chloroplasts chlorophyll-containing organelle found in phototrophic eukaryotes relatively large; number of chloroplasts vary double membrane Inner membrane surrounds stroma, which contains large amounts of RubisCO (key for Calvin cycle that converts CO2 to organics). thylakoids: flattened membrane discs contain chlorophyll and ATP synthetic components, form proton motive force 2.15 Mitochondria, Hydrogenosomes, and Chloroplast Organelles and Endosymbiosis Endosymbiotic hypothesis: Mitochondria and chloroplasts descended from respiratory and phototrophic bacterial cells, respectively, associating with nonphototrophic eukaryal hosts. Free-living symbionts became part of eukaryotic cell. Evidence: mitochondria, hydrogenosomes, chloroplasts contain circular DNA genomes and ribosomes. 2.16 Other Eukaryotic Cell Structures Endoplasmic reticulum (ER) network of membranes continuous with nuclear membrane two types (rough and smooth) Rough contains attached ribosomes; smooth does not. Smooth ER participates in the synthesis of lipids and carbohydrate metabolism. Rough ER is a major producer of glycoproteins and new membrane material. Golgi complex stacks of cisternae (membrane- bound sacs) functioning in concert with the ER modifies products of the ER destined for secretion 2.16 Other Eukaryotic Cell Structures Lysosomes membrane-enclosed compartments contain digestive enzymes used for hydrolysis of food degrade and recycle damaged cell components separate lytic activity from cytoplasm 2.16 Other Eukaryotic Cell Structures Cytoskeleton: internal structural support microtubules (Figure 2.52a and b) composed of α- and β-tubulin maintain cell shape and motility; move chromosomes and organelles microfilaments (Figure 2.52c) polymers of actin maintain cell shape; involved in amoeboid motility and cell division intermediate filaments keratin proteins maintain cell shape and position organelles 2.16 Other Eukaryotic Cell Structures Flagella and cilia (Figure 2.53) Cilia are short flagella. organelles of motility that allow cells to move by swimming structurally distinct from prokaryotic flagella and do not rotate; instead whip (flagella) or beat in synchrony (cilia) bundle of nine pairs of microtubules surrounding a central pair of microtubules Dynein is attached to the microtubules and uses ATP to drive motility.