Cell Surface Structures and Inclusions PDF

LESSON: 2 CELL SURFACE STRUCTURES AND INCLUSIONS  Capsules and Slime Layers  Sticky coat of polysaccharide formed outside the cell envelope  Mediate attachment, protection, alter diffusive environment  When binding to solid surfaces, they form a thick layer = biofilm  Capsule...

LESSON: 2 CELL SURFACE STRUCTURES AND INCLUSIONS  Capsules and Slime Layers  Sticky coat of polysaccharide formed outside the cell envelope  Mediate attachment, protection, alter diffusive environment  When binding to solid surfaces, they form a thick layer = biofilm  Capsule  organized in tight matrix, excludes small particles, tightly attached to cell  Visible in light microscopy, treated with India ink (stains only the background, cannot penetrate the capsule)  Also seen in electron microscopy  Microcapsule - if the layer is too thin to be seen by light microscope  Slime layer  Easily deformed, loosely attached, include particles, more difficult to see, easily detected in colonies, abundant and embedded in common matrix (ex: lactic acid Leuconostoc)  Also stained by india ink  Fimbriae, Pili, and Hami  Pili  Thin filamentous structures made of protein extend from surface of cell  Enable bacterial cells to stick to surfaces  Form pellicles in liquid surfaces  Form biofilms in solid surfaces  All gram-negative bacteria produce pili Types of Pili:  Conjugative pili - facilitate genetic exchange during conjugation (cell-to-cell attachment)  Electrically conductive pili - conduct electrons toward or away the cell, plays role in metabolism  Type IV pili - facilitate adhesion, support twitching motility (allows cells to move along solid surface)// assist in infectivity of some pathogens: cholera, gonorrhea, strep throat, scarlet fever  F pilus (sex pilus) - path of entry of genetic material during mating  Fimbriae  Short pili that mediate attachment  Hami/ Hamus  Resembles a tiny grappling hook  Present in SM1 group (Archaea)  Attach cells to surfaces to form a networked biofilm (more efficient trap for scarce nutrients present) and to each other CELL INCLUSIONS  Often visible with light microscope  Carbon Storage Polymers  Poly-β-hydroxybutyric acid (PHB)  most common inclusion body in prokaryotes, monomer polymerized by ester linkage   Monomers are usually hydroxybutyrate (C4)  poly-β-hydroxyalkanoate (PHA) - more generic term, synthesized when there is an excess carbon// broken down for energy (carbon)  Glycogen  Polymer of glucose  Reservoir of carbon and energy  Also produced when there is an excess carbon  Resembles starch (storage reserve of plants)  Polyphosphate, Sulfur, and Carbonate Minerals  Polyphosphate  Granules in accumulation of inorganic phosphate  Formed when phosphate is in excess  Can be a source of phosphate for nucleic acid and phospholipid biosynthesis  Sulfur  Sulfur bacterias: organisms that oxidize reduced sulfur compounds  Discovered by Sergei Winogradsky  Also used for energy metabolism  Carbonate materials  Formed by filamentous cyanobacteria on external & internal surface of cell  Gloeomargarita : forms granules of bentonite (carbonate material) containing barium, strontium, magnesium  Biomineralization - microbiological process of forming minerals  Gas Vesicles  Confer buoyancy allows floating  Allow cells to position themselves in regions of water column best suited for their metabolism  Conical-shaped  Impermeable to water and solutes, permeable to gases  Gas vacuoles - clusters of vesicles  Blooms  Gas-vesiculate microbes that form massive accumulations  Common on near lake surfaces where sunlight is intense  Magnetosomes  Allows bacteria to orient themselves in a magnetic field thru magnetic dipole  Biomineralized particles of magnetic iron oxides magnetite or greigite  Magnetotaxis - process of migrating along Earth's magnetic field lines  Spiked-shaped - most common  Could also be square/rectangular ENDOSPORES  Specialized spores  Endo = within  Highly differentiated dormant cells that can tolerate harsh conditions  Dormant stage of bacteria life cycle: vegetative cell -> endospore -> vegetative cell   They only grow when conditions become favorable (high alanine and nutrients)  Easily dispersed by wind, water, animal gut (widely distributed)  Produced only by 2 groups: 1. Bacillales (2) Clostridiales  Both are gram positive  Causes food spoilage and foodborne diseases + tetanus & botulism  Formation and Germination  Sporulation  process of cellular differentiation that results in endospore formation  Triggered when nutrient becomes limiting  Germination  Conversion of endospore back to a vegetative cell  Triggered by availability of nutrients like amino acids and sugars  Occurs in 3 steps: activation, germination, outgrowth  Structure and Features  Visible by light microscope as strongly refractile structures  Impermeable to most dyes  Malachite green stain - used infused to spore with steam  Contains many layers absent from vegetative cell (seen in electron):  Core - innermost, contains DNA and ribosomes, from cytoplasm of vegetative cell  Inner membrane - from cyto membrane of vege cell  Cortex - 2 peptidoglycan layers  Outer membrane - special membrane formed during sporulation  Endospore coat - composed of layers of spore-specific proteins  Exosporium (not all) - outer proteinaceous layer  Dehydration of the core - ultimate reason for endospore toughness// caused by accumulation on calcium-dipicolinic acid  Small acid-soluble spore proteins (SASPs)  made during sporulation  2 functions: 1. Bind tightly to DNA in the core 2. Protect from damage of UV radiation, dry heat, desiccation  The Sporulation Cycle Asymmetric cell division: The sporulating cell divides asymmetrically, producing a larger mother cell and a smaller forespore. Engulfment: The mother cell engulfs the forespore, and the forespore becomes a cell within the mother cell cytoplasm. Cortex formation: The cortex, a peptidoglycan layer, is formed between the inner and outer forespore membranes. Coat formation: The coat, a proteinaceous layer, is formed on the exterior of the spore. Maturation: The spore matures and becomes resistant to environmental stress. Germination: The spore germinates, and the vegetative cell emerges. CELL LOCOMOTION  Motility  allows cell to reach different parts of their environment  2 major types of prokaryotic movement: swimming & gliding  Taxis - how motile cells are able to move towards or away from stimuli  Flagella & Archaella - tiny rotating machines that function to push or pull the cell through a liquid Flagellum / Flagella  Provides swimming motility for bacterias  Long, thin appendages  Observed by light microscopy or electron microscopy  Tuft - group of flagella  Do not rotate in constant speed, it depends on the proton motive force  Fastest speed - 60 cell-lengths/s  Not straight but helical  In order to change direction, they also need to change the direction of their rotation (ex: CW - CCW)  Can be anchored to a cell in different locations: Polar flagellation - attached at one or both ends - they spin around moving from place to place - tuft at one end is called lophotrichous seen by dark-field or phase contrast microscopy - tuft at both ends is called amphitrichous Peritrichous flagellation - flagella inserted around cell surface - move slowly in a straight line  Structure of Flagella -Filament - composed of many copies of proteins called flagellin - hook - wider region at the base, single type of protein, connects the filament to the flagellum motor - flagellum motor - reversible rotating machine, anchored in cytoplasmic membrane and cell wall Central rod - passes thru a series of rings L-ring - outer ring in outer membrane P-ring - in peptidoglycan layer MS and C Rings - third set of rings, within cytoplasmic membrane and cytoplasm Mot proteins - series if proteins that surrounds the inner rings, in cyto membrane and peptidoglycan// sets the proton flow rate Fli proteins - set of proteins, functions as motor switch, reversing the direction of rotation of the flagella Flagellum motor has 2 components:   Rotor - consists of central rod, L, P, C, MS rings// make up the flagellar basal body  Stator - consists of the mot proteins that surround the rotor and function to generate torque  Flagellar Synthesis  Filament grows from tip  MS & C rings are synthesized -> inserted to cyto membrane -> followed by other rings -> early hook -> cap -> late hook -> filament synthesis  Flagellin flows through the hook to form filament  Cap - a protein present at the end of growing flagellum to guide them into position Archaellum / Archaella  Present in archaea  Rotate same as flagella  7-12 genes encode the major proteins that make up it  Usually studied in Halobacterium (salt-loving)  The speed is slow due to smaller diameter than flagella (smaller diameter = reduced torque)  Methanocaldococcus can swim at 500 cell lengths per sec  Embedded in archaeal cell wall and cyto membrane  Hydrolysis of ATP drives the rotation (not proton motive force)  Capable of Clockwise and Counterclockwise rotations CILIA  For cell motility and sweeping food organisms into oral cavity  Energy from ATP  Beating caused by intraciliary excitation followed by interciliary conduction  4 types of ciliary movements 1. Pendulus ciliary movement  Carried out in single plane  Occurs in paramecium (those who have rigid cilia) 2. Unciform ciliary movement  Hook-like movement  Metazoan cells 3. Infundibuliform ciliary movement  Occurs due to rotary movement of cilium and flagellum 4. Undulant movement  Waves of contraction CYTOSKELETAL COMPONENTS  Intermediate filaments  Rope-like assemblies of fibrous polypeptides  Support the nuclear envelope and plasma membrane  Microtubules  Small, hollow cylinders made of tubulin protein  Composed of a-tubulin and b-tubulin MICROBIAL LOCOMOTION  Internal Movement (Cytoplasmic Streaming)  Organelles move within cytoplasm  Governed by actin filaments and cytoskeleton  Facilitates distribution of nutrients and materials inside the cell  External Movement (Motility)  Involves specialized organelles for locomotion  Pseudopodia - temporary extensions of the cell body  Cilia - short, haiir-like structures  Flagella - long, whip-like SURFACE MOTILITY  All surface motility are considerably slower than swimming motility  Twitching motility  Requires type IV pili which extend from one pole of the cell, attach to the surface, then retract to pull the cell forward  Energy is from ATP hydrolysis  Allows cells to move in groups  Facilitated by type IV pili and secretion of extracellular polysaccharides  Observed in Pseudomonas & myxobacteria  Myxobacteria exhibit 2 forms: social motility (caused by twitching) & adventurous motility (gliding)  Gliding motility  Smooth motion along the axis of the cell without the aid of external propulsive structures  Continuous form of movement in a helical track  Gliding bacterias are typically filamentous or rod shaped  No gliding archaea known  Best observed in myxococcus and flavobacterium  Gliding motors - rotary motors driven by proton motive force which are operationally similar to flagellar motors CHEMOTAXIS  Taxis - directed movement toward a stimuli  Chemotaxis - response to chemicals  Observed in swimming bacterias (E. coli) by observing altering of rotations of flagellum  Chemotaxis in Peritrichously Flagellated Bacteria  Runs & tumble  Runs - cell is swimming forward in a smooth fashion  Tumble - when the cell stops and jiggles about randomly  In the absence of attractant, the cell moves about its environment in random fashion through a series of runs and tumbles  In the present of attractant, the cell moves toward the attractant but not DIRECTLY but thru a biased random walk  Chemoreceptors - sense attractants and repellents  Chemotaxis in Polarly Flagellated Bacteria  Similar to peritrichously flagellated cells but they do not tumble but instead they swim backwards  Biased random walk - can navigate effectively through their environments toward conditions that favor growth and away from those that could inhibit growth or otherwise cause harm  Other Forms of Taxis  Phototaxis - response to light  Observed in filamentous cyanobacteria  Photoreceptor - senses light  Scotophobotaxis occurs when a phototrophic bacterium happens to swim into darkness outside the illuminated field of view of the microscope  mechanism to prevent phototrophic cells from swimming away from a lighted zone into darkness  Osmotaxis - response to ionic strength  Hydrotaxis - response to available water  Aerotaxis - response to O2, ex: Microaerophiles  Magnetotaxis - magnetotactic bacteria found where O2 are low  do not actually exhibit directed motility toward magnetic fields but instead are exhibiting aerotaxis