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CELL STRUCTUREAs any beginning student of microbiology
knows, bacteria come in three basic models:spherical (coccus), rod (bacillus), and spiral
(spirillum).
They do not possess a membrane boundnucleus as do eukaryotic microorganisms;therefore, they are prokaryotic.
In addition to these basic types of bacteria,there are other more specialized formsdescribed as budding, sheathed, and mycelial.Figure 1 1 presents a schematicrepresentation of a typical (meaning E. coli )
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S-layer, outer membrane and cell wallS-layer
A protein or glycoprotein layer is found on the surface of all prokaryoticcells except mycoplasma. This is called an S-layer (Figure 2.3).
Functions of the S-layer are (1) protection from toxic compounds,(2) adhesion to solid surfaces, (3) a phage receptor, (4) a physicalstructure to maintain cell morphology, and (5) a binding site for certain extracellular enzymes.
Out er membraneG ram-negative bacteria are more resistant to lysozyme,
hydrolytic enzymes, bile salts and hydrophobic antibiotics thanG ram-positive bacteria. These properties are due to the presencof the outer membrane in G ram-negative bacteria
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The outer membrane (OM) is different in structure from thecytoplasmic membrane (CM).
The CM consists of phospholipids while lipopolysaccharide(LPS) forms the outer leaflet of the OM with the inner leaflet composedof phospholipids.
Lipopolysaccharide (LPS) consists of three components: lipid A ,core polysaccharide and repeating polysaccharide (referred to as O-antigen).
Lipopolysaccharide LPScontains unique sugarsL-glycero-D-mannoheptose
(Hep) and 2-keto-3-deoxyoctonate (KDO), and raresugars such as abequose ( A be)and colitose (Col).G alactose ( G al), glucose ( G lc),mannose (Man) and rhamnose
(Rha) can also be present.
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Figure 2:3 Cell surfacestructures of prokaryotic cells.(a) Archaea, (b) Gram-positive
bacteria, (c) Gram-negativebacteria.S, S-layer; CM, cytoplasmicmembrane; CW, cell wall; OM,outer membrane; PG,
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Lipid A structure in Salmonellatyphimurium.
3-hydroxy fatty acids are bound toglucosamine and the core polysaccharide is linked to carbon 6as indicated by the dotted line.
The structure of the outer membrane of G ram negative
bacteria.
Omp A , outer membrane protein A .
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C ell Wall.In 1884, the Danish investigator Christian G ram devised a
differential stain based on the ability of certain bacterial cells to retainthe dye crystal violet after decoloration with 95% ethanol.With a few exceptions, prokaryotic cells have a cell wall that
provides the physical strength to maintain their shapes. Murein is themain component of the cell wall of bacteria.
The cell wall in G ram-negative bacteria is much thinner than inG ram-positive bacteria, which have a complex cell wall with other polymers and do not possess an outer membrane A s shown in theschematic drawings in Figure 1-3.
Cells that retained the stain were called gram positive.
The gram-positive cell surface has two major structures: thecell wall and the cell membrane. The cell wall of gram-positive cellsis composed of multiple layers of peptidoglycan, which is a linear
polymer of alternating units of N-acetylglucosamine (NAG ) and N-acetylmuramic acid (N A M).
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A short peptide chain is attached to muramic acid.G ram-positive bacteria do not have an outer membrane but have a
much thicker cell wall containing teichoic acid, lipoteichoic acid and
lipoglycan in addition to murein.A common feature in bacterial cell walls is cross-bridging between
the peptide chains.Other componentsfor example, lipoteichoic acid (only present in
gram-positive organisms)are synthesized at the membrane surfaceand may extend through the peptidoglycan layer to the outer surface.
The peptidoglycan layer of a gram-negative cell is generally a singlemonolayer.
A n outer membrane surrounding the gram-negative cell is composedof phospholipids, lipopolysaccharides, enzymes, and other proteins,including lipoproteins.
The space between this outer membrane and the inner membrane is
referred to as the periplasmic space (Fig. 1-2).
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M embranes:-The cytoplasmic membrane of both gram-positive and gram-
negative cells is a lipid bilayer composed of phospholipids,glycolipids, and a variety of proteins.
The proteins in the cytoplasmic membrane may extend through itsentire thickness.
Some of these proteins provide structural support to the membranewhile others function in the transport of sugars, amino acids, and other metabolites.
The cytoplasmic membrane mediates not only thesefunctions but also other important physiological activities.These include solute transport , oxidative phosphorylation
through electron transport, photosynthetic electrontransport in photosynthetic prokaryotes , maintenanceof electrochemical gradients and A TP synthesis , motility, synthesis of cell surface structures and protein secretion .
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The cytoplasmic membrane consists of phospholipid (3550%) and protein (5065%). The phospholipid is responsible for the isolation
property of the membrane with the various proteins being involved in threst of the membrane functions.
Phospholipid forms both inner and outer leaflets of the cytoplasmicmembrane, but the membrane is asymmetrical due to proteins presentin the membrane. The phospholipid bilayer membrane is permeable to
hydrophobic solutes and water but not to charged solutes and polymers.Membrane proteins transport these in and out of the cell. Thoughwater can diffuse through the membrane, the diffusion rate is too low
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CytoplasmThe cytoplasm refers to everything inside the cytoplasmic membrane.Cells are classified as prokaryotes or eukaryotes depending
on the possession of a nucleus. Eukaryotic cells have well-developedintracellular organelles such as mitochondria, chloroplasts andendoplasmic reticulum in addition to the nucleus. With only a fewexceptions, prokaryotic cells do not have subcellular organelleswithin the cytoplasm.
Prokaryotic cytoplasm contains DN A , ribosomes, proteins, RN A ,salts and metabolites and is viscous due to the high concentration of macromolecules
Some of these macromolecules form aggregates, while others aresoluble. The soluble part is called the cytosol.
Proteins in the cytoplasm are in high concentration and can interactwith each other to form a kind of network.
The enzymes involved in a particular metabolic process are adjacentfor their required interaction.
The term metabolon has been proposed to describe such a set of enz mes and their cofactors involved in such a fashion.
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C aps u les.Some bacterial cells are covered with capsule or a slime layer
Capsules are composed of either polysaccharides (high molecular-weight polymers of carbohydrates) or polymers of amino acids called
polypeptides (often formed from the D- rather than the L-isomer of anamino acid).
The capsule of Streptococcus pneumoniae type III is composed of glucose and glucuronic acid in alternating -1, 3- and -1, 4- linkages.
This capsular polysaccharide, sometimes referred to as pneumococcal polysaccharide, is responsible for the virulence of the
pneumococcus.
B acillus anthracis, the anthrax bacillus, produces a polypeptidecapsule composed of D-glutamic acid subunits, whichis a virulence factor for this organism.
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C aps u les of Streptococcus pneumoniae
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Flagella and piliMotile prokaryotic cells have an appendage called a flagellum
(plural, flagella) involved in motility.motility is accomplished by means of simple strands of protein (flagellin) woven into helical organelles called flagella.
The bacterial flagellum consists of three parts. These are a basal body, a hook and a filament (Figure ).
The basal body is embedded in the cytoplasmic membrane and cellsurface structure and connected to the filament through the hook.
The number and location of flagella vary depending on the bacterialspecies.
A similar but smaller structure, the fimbria (plural, fimbriae).Fimbriae are not involved in motility and are composed of proteins.Fimbriae consist of a major protein with minor proteins called
adhesins that facilitate bacterial attachment to surfaces by recognizingthe appropriate receptor molecules.
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The fimbria, also known as the pilus (plural, pili), is observed inmany G ram-negative bacteria but rarely in G ram-positive bacteria.
Fimbriae have been proposed as the fibrils that mediate attachmentto surfaces. For this reason, the term pilus should be used only todescribe the F-pilus, the structure that mediates conjugation.
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