BACTERIAL STRUCTURE. The gross morphological characteristics of bacterial cells… Size, Shape,...
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Transcript of BACTERIAL STRUCTURE. The gross morphological characteristics of bacterial cells… Size, Shape,...
BACTERIAL STRUCTURE
• The gross morphological characteristics of bacterial cells…
• Size, Shape, Structure, Arrangement– Pairs, Clusters, Trichomes, Filaments
Size
• Bacteria are very small: ~ 0.5 to 1.0 μm in diameter.
• The surface area to volume ratio of bacteria is exceedingly high comparitively.
• A relatively large surface through which nutrients can enter (or waste products leave) compared to a small volume of cell substance to be nourished.
Unusually high growth rate and metabolism of bacteria.
• Because of the high surface area-volume ratio, the mass of cell substance to be nourished is very close to the surface; therefore, no circulatory mechanism is needed to distribute the nutrients that are taken in.
Shape and Arrangement
• Shape determined by its rigid cell wall
• Typical bacterial cells are– Spherical (cocci; coccus)– Straight rods (bacilli; bacillus)– Helically curved Rods (spirilla; spirillum)
• Some cells are “Pleomorphic”
Staphylococcus
Lactobacillus
Aquaspirillum
Pleomorphic Arthrobacter
Characteristic arrangements of cocci
• Diplococci: – Cells divide in one plane and remain attached
predominantly in pairs. • Streptococci:
– Cells divide in one plane and remain attached to form chains.
• Tetracocci: – Cells divide in two planes and characteristically form
groups of four cells.• Staphylococci:
– Cells divide in three planes, in an irregular pattern, producing “bunches” of cocci.
• Sarcinae: – Cells divide in three planes, in a regular pattern,
producing a cuboidal arrangement of cells
Diplococci
Streptococci (Chains)
Tetracocci
Staphylococci (Bunches)
Sarcinae (Cuboidal Packets)
Arrangements in Bacilli
• Diplobacilli: Pairs of Bacilli• Streptobacilli: Chain formation. Eg. B.subtilis• Trichomes: Eg. Beggiotoa and Saprospira sp.
– Similar to chains but with much larger area of contact between the adjacent cells
• Pallisade: Eg. Corynebacterium diphtheriae– Cells are lined side by side like matchsticks and at
angles to one another.
• Hyphae and Mycelia: Eg. Streptomyces species– Long, branched, multinucleate filaments called
hyphae which collectively form a mycelium– Note: Hyphae and mycelium are also commonly
applied to the filaments formed by fungi.
Saprospira
Corynebacterium diphtheriae
Arrangements in Curved bacteria
– Usually curved with 1 twist. – Vibrioid shape: Bacteria with less than 1
complete twist– Helical Shape: Bacteria with one or more
complete turns have a helical shape. Eg. Spirilla are rigid helical bacteria, Spirochetes are highly flexible.
Uncommon Shapes & Arrangements
• Pear-shaped cells (e.g.. Pasteuria)
• Lobed spheres (e.g.. Sulfolobus)
• Rods with squared rather than the usual hemispherical ends (e.g. B.anthracis)
• Disks arranged like stacks of coins (e.g.. Caryophanon)
• Rods with helically sculptured surfaces (e.g.. Seliberia)
In bacteria,
STRUCTURES
EXTERNAL
TO THE CELL WALL
FLAGELLA• Singular: Flagellum• A long, slender, helical appendage from the cell body,
composed of microtubules, protruding through the cell wall
• In prokaryotes, responsible for swimming motility to propel the cell by beating in a whip-like motion
• In larger animals, they often serve to move fluids along mucous membranes such as the lining of the trachea
• 0.01 to 0 02 μm in diameter• Location on the cell varies depending on the bacterial
species• At polar region(s): At one or both ends of the bacterium• At lateral regions: Along the sides of the bacterium
Monotrichous: A single polar flagellum Eg.: Pseudomonas aeruginosa
Lophotrichous: A cluster of polar flagellaEg.: Psedomonas fluorescens
Amphitrichous: Either single or clusters, at both cell poles.
Eg.: Aquaspirillum serpens
Peritrichous: Surrounded by lateral flagellaEg.: Salmonella typhi
• A flagellum is composed of three parts:– A basal body associated with the cytoplasmic
membrane and cell wall– A short hook– A very long helical filament
• Some Gram-negative bacteria have a sheath surrounding the flagellum: this sheath is continuous with the outer membrane of the Gram-negative cell wall.
• The hook and filament are composed of protein monomers arranged helically
• The protein of the filament is known as flagellin.
• The flagellar motor is driven by the protonmotive force. i.e.. the force derived from the electrical potential and the hydrogen-ion gradient across the cytoplasmic membrane.
• The concentration of cGMP (cyclic guanosine 3’,5’- phosphoric acid) within the cell governs the direction in which the rotation occurs.
Bacteria having polar flagella swim in a
back-and-forth fashion;
They reverse their direction of swimming
by reversing the direction of flagellar
rotation.
Motility without flagella
• No external flagella in certain helical bacteria (spirochetes)
• Have flagella-like structures located within the cell.
• Just beneath the outer cell envelope.• Called Periplasmic flagella or Axial fibrils or
Endoflagella. • Other helical bacteria called spiroplasmas lack
any apparent organelles for motility, even periplasmic flagella.
• Yet, swims in viscous media; Mechanism unknown.
Chemotaxis• Capability of directed swimming toward or away from
various chemical compounds: Bacterial chemotaxis.• Swimming toward a chemical is termed positive
chemotaxis; swimming away is negative chemotaxis.• A change in the concentration of the chemical with
time, i.e., a temporal gradient.• Gradients sensed by means of protein
chemoreceptors, located on the cytoplasmic membrane
• Receptors, specific for different attractants and repellents.
• Continuous comparison with its immediate environment with the environment it had experienced a few moments ago.
Phototaxis
• Exhibited by phototrophic bacteria – Positive: Motion towards increasing light
intensities– Negative: Motion repelled by decreasing light
intensities.
Magnetotaxis
• Exhibits directed swimming in response to the earth’s magnetic field or to local magnetic fields
• Eg.: Aquaspirillum magnetotacticum• Due to a chain of magnetite inclusions (magnetosomes)
within the cell• Cell becomes oriented as a magnetic dipole.• Found in regions where there is a downward inclination
of the Earth’s magnetic field in the regions where these bacteria have been found
• May serve to direct the cells downward in aquatic environments toward oxygen-deficient areas more favorable for growth.
PILI (FIMBRIAE)
• Pilus: Hollow, nonhelical, filamentous appendages that are thinner, shorter & and more numerous than flagella
• Not for motility• Found on nonmotile as well as motile species• Different types of pili
– F pilus (or sex pilus): Serves as the port of entry of genetic material during bacterial mating
– For allowing pathogenic bacteria to attach to epithelial cells lining the respiratory, intestinal, or genitourinary tracts, in human infection.
– To prevent the bacteria from being washed away by the flow of mucous or body fluids and permits the infection to be established.
CAPSULE
• A viscous substance forming an envelope around the cell wall
• Consist of a mesh or network of fine strands.
• May or may not be soluble in water.• Microcapsule: If the layer is too thin to be
seen by light microscopy• Slime: If it is abundant
– Many cells embedded in a common matrix.
Function(dependent on the bacterial spp)
• Provides protection against temporary drying by binding water molecules
• Blocks attachment of bacteriophages• Antiphagocytic i.e., they inhibit the engulfment of
pathogenic bacteria by WBCs, so, contributes to virulence
• Promote attachment of bacteria to surfaces– Eg.: Streptococcus mutans, produces dental caries; firmly
adheres to the smooth surfaces of teeth because of its secretion of a water-insoluble glucan
• May promote stability of bacterial suspension by preventing the cells from aggregating and settling out– Only if capsules are composed of compounds with an electrical
charge– Cells bearing similarly charged surfaces repel.
• Capsules are usually composed of polysaccharides.
• If composed of a single kind of sugar: “Homopolysaccharides”– Eg.: Synthesis of glucan (a polymer of glucose) from
sucrose by S. mutans • If composed of several kinds of sugars:
“Heteropolysaucharides”– Eg.: Capsule of Klebsiella pneumoniae
• Some polypeptide capsules– Eg.: Capsule of Bacillus anthracis – a polymer of Glu
SHEATH
• Some bacterial spp. esp.from freshwater and marine environments
• Formation of Chains or trichomes enclosed by a hollow tube called a Sheath.
• May sometimes contain ferric or manganese hydroxides.
Prosthecae
• Semirigid extensions of the cell wall and cell membrane
• Present in aerobic bacteria from freshwater and marine sources– Eg. Caulobacter (1 prostheca), Stella &
Ancalomicrobium (many)• Increases the surface area of the cells for
nutrient absorption• May form a new bud at the end of a prostheca• May aid in attachment to surfaces using an
adhesive substance at the end of a prostheca.
CELL WALL
• Surrounding the cytoplasmic membrane• Highly rigid structure giving shape to the
cell• Prevents the cell from expanding and
eventually bursting because of uptake of water
• Hypotonic environments (environments having a lower osmotic pressure than exists within the bacterial cells)
• Among the ordinary or typical bacteria (eubacteria)– Gram-negative species: Generally thinner
walls (10 to 15 nm)– Gram-positive species: Thicker walls (20 to 25
nm)
Composition:
• Shape is determined by peptidotglycan / murein)
• An insoluble, porous, cross-linked polymer – strong and rigid.
• A polymer of N-acetylglucosamine, N-acetylmuramic acid, L-Ala, D-Ala, D-Glu and a diamino acid.
• In archaeobacteria– Possess cell walls, but NO peptidoglycan– Usually composed of proteins, glycoproteins,
or polysaccharides.
•
Gram-positive bacteria
• Have a much greater amount of peptidoglycan.
• Accounts for > 50% of the dry weight of the wall of some Gram-positive species.
• Teichoic acid. – Eg.: Staphylococcus aureus and Streptococcus
faecalis– Acidic polymers of ribitol phosphate or glycerol
phosphate, covalently linked to peptidoglycan– Binds magnesium ions.– Protects bacteria from thermal injury.
• Also, polysaccharides, covalently linked to the peptidoglycan. Eg.: Streptococcus pyogenes.
• Most Gram-positive bacteria contain very little lipid.– Exceptions: Mycobacterium, Corynebacterium
Gram-negative bacteria• More complex than those of Gram-positive
bacteria.• A 8 nm-thick outer membrane surrounds a thin
underlying layer of peptidoglycan.• Rich in lipids (11 to 22% of cell wall dry weight).• Serves as an impermeable barrier to prevent the
escape of important enzymes, such as those involved in cell wall growth, from the periplasmic space.
• Serves as a barrier to external chemicals and enzymes that may damage the cell.
• Outer membrane held to peptidoglycan by means of Braun’s lipoprotein (5-7 nm thick layer)
• A bilayered structure consisting mainly of phospholipids, proteins & LPS.
• The LPS has toxic properties: Endotoxin.• Occurs only in the outer layer of the outer membrane• Composed of 3 covalently linked parts
– Lipid A, firmly embedded in the membrane– Core polysaccharide, located at the membrane surface– Polysaccharide O antigens - extend from the membrane surface
into the surrounding medium.
• Also serve as receptors for bacteriophage attachment.
• Porins: Membrane-spanning channel protein – To allow nucleosides, mono and oligosaccharides, peptides & amino acids, to pass across
• Different porins, specific for different classes of small molecules
In bacteria,
STRUCTURES
INTERNAL
TO THE CELL WALL
Cytoplasmic Membrane:
• Immediately beneath the cell wall.• ~ 7.5 nm thick• Composed primarily of phospholipids (about 20-
30 %) & proteins (about 60-70 %). • A hydrophobic barrier to penetration by most
water-soluble molecules.• Contains various enzymes involved in
respiratory metabolism and in synthesis of capsular and cell-wall components.
• Impermeable to protons (H+ ions).
• Phospholipids form a bilayer in which most of the proteins are held (integral proteins)
• Other proteins are only loosely attached (peripheral proteins)
• Lipid matrix of the membrane has Fluidity, allowing the components to move around laterally.
• ln eubacteria, the phospholipids are phosphoglycerides: – Straight-chain fatty acids are ester-linked to
glycerol.
• ln archaeobacteria, the phospholipids are polyisoprenoid branched-chain lipids:– Long-chain branched alcohol (phytanols) are
ether-linked to glycerol.
• Proteins, synthesized within the cell, passing across the cell membrane: – Protein components of cell walls – Porins,
Lipoproteins– Exocellular enzymes – penicillinases,
proteinases and amylases. – Proteins remaining back – Cytochromes,
Dehydrogenases
PROTOPLAST• Portion of a bacterial cell consisting of the
cytoplasmic membrane and the cell material bounded by it.
• Can be prepared from Gram-positive bacteria – By treating the cells with an enzyme such as
lysozyme, which selectively dissolves the cell wall– By culturing the bacteria i.p.o. an antibiotic such as
penicillin, which prevents the formation of the cell wall.
• To protect the organisms from bursting. • Bursting can be prevented by preparing
protoplasts in an isotonic medium, i.e., in a medium that has an osmotic pressure similar to that of the protoplast.
SPHEROPLAST
• Round, osmotically fragile forms of Gram-negative bacteria
• Prepared by similar procedures
• Has two membranes, the cytoplasmic membrane of the protoplast plus the outer membrane of the cell wall.
Membranous Intrusions
• Mesosomes – Membrane invaginations as Convoluted
tubules and vesicles.– Central mesosomes: Penetrate deeply;
Located near the middle of the cell; Attached to the cell’s nuclear material; Involved in DNA replication and cell division
– Peripheral mesosomes: Shallow penetration; No association with nuclear material: Involved in export of exocellular enzymes.
Intracellular membrane systems
– In Methane-oxidizing bacteria, in some chemoautotrophic bacteria & phototrophic bacteria.
– To increase surface area for various metabolic activities.
– Eg.: Thylakoids in Cyanobacteria.
CYTOPLASM
• The area enclosed by the cvtoplasmic membrane may be divided into – Cytoplasmic area, granular in appearance
and rich in the macromolecular RNA-protein bodies: Ribosomes
– Chromatinic area, rich in DNA– The fluid portion with dissolved substances.
• Volutin / Metachromatic granules: – Composed of polyphosphate. – Serves as a reserve source of phosphate.
• Poly-β-hydroxybutyrate. (PHB):– In aerobic bacteria– Serves as a reserve carbon and energy source.
• Gas vacuoles:– Bacteria living in aquatic habitats– To provide buoyancy.
SPORES
• A metabolically dormant form which, under appropriate conditions, can undergo germination and outgrowth to form a vegetative cell.
• Certain species of bacteria produce spores. – Within the cell (endospores) or – External to the cell (exospores).
ENDOSPORES• Thick-walled, highly refractile bodies that are produced
one per cell • Formed by Bacillus, Clostridium, Sporosarcina,
Thermoactinomyces• Extremely resistant to desiccation, staining, disinfecting
chemicals, radiation, and heat. • Average degree of heat resistance of endospores: upto
80°C for 10 min.• Dehydration occurs: Most water in the developing spore
is expelled• Contain abundant dipicolinic acid (DPA) in combination
with large amounts of calcium• Calcium-DPA complex – a role in the heat resistance of
endospores.
EXOSPORES
• Cells of the methane-oxidizing genus Methylosinus
• Form spores external to the vegetative cell, by budding at one end of the cell.
• Desiccation- and heat-resistant.
• No DPA.
• Actinomycetes form branching hyphae.• From hyphal tips, spores form by crosswall
formation (septation). • Spores develop, singly or in chains.
– Spores contained in an enclosing sac called Sporangium: Sporangiospores
– Else, Conidiospores (or conidia).The spores do not have the high heat resistance of endospores, but they can survive long periods of drying.
CYSTS
• Dormant, thick-walled, desiccation-resistant forms that develop by differentiation of a vegetative cell and which can later germinate under suitable conditions
• Resemble endospores• Structure and chemical composition are different• Do not have the high heat resistance of
endospores. • Eg.: cysts of Azotobacter genus.