bacteria cell structure

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• CELL STRUCTURE
• As 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-bound nucleus as
  do eukaryotic microorganisms therefore, they are
  prokaryotic.
• In addition to these basic types of bacteria,
  there are other more specialized forms described
  as budding, sheathed, and mycelial. Figure 1-1
  presents a schematic
• representation of a typical (meaning E. coli )
  bacterial cell.

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• S-layer, outer membrane and cell wall
• S-layer
• A protein or glycoprotein layer is found on the
  surface of all prokaryotic
• cells 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 physical
• structure to maintain cell morphology, and (5) a
  binding site for
• certain extracellular enzymes.
• Outer membrane
• Gram-negative bacteria are more resistant to
  lysozyme, hydrolytic enzymes, bile salts and
  hydrophobic antibiotics than Gram-positive
  bacteria. These properties are due to the
  presence of the outer membrane in Gram-negative
  bacteria

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• The outer membrane (OM) is different in structure
  from the cytoplasmic membrane (CM).
• The CM consists of phospholipids while
  lipopolysaccharide
• (LPS) forms the outer leaflet of the OM with the
  inner leaflet composed of phospholipids.
• Lipopolysaccharide (LPS) consists of three
  components lipid A,
• core polysaccharide and repeating polysaccharide
  (referred to as O-antigen).

Lipopolysaccharide LPS contains unique
sugars L-glycero-D-mannoheptose (Hep) and
2-keto-3- deoxyoctonate (KDO), and rare sugars
such as abequose (Abe) and colitose
(Col). Galactose (Gal), glucose (Glc), mannose
(Man) and rhamnose (Rha) can also be present.
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Figure 23 Cell surface structures of prokaryotic
cells. (a) Archaea, (b) Gram-positive
bacteria, (c) Gram-negative bacteria. S, S-layer
CM, cytoplasmic membrane CW, cell wall
OM, outer membrane PG,
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Lipid A structure in Salmonella
typhimurium. 3-hydroxy fatty acids are bound
to glucosamine and the core polysaccharide is
linked to carbon 6 as indicated by the dotted
line.
The structure of the outer membrane of Gram
negative bacteria. Omp A, outer membrane protein
A.
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• Cell Wall.
• In 1884, the Danish investigator Christian Gram
  devised a differential stain based on the ability
  of certain bacterial cells to retain the 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 the main
  component of the cell wall of bacteria.
• The cell wall in Gram-negative bacteria is much
  thinner than in Gram-positive bacteria, which
  have a complex cell wall with other polymers and
  do not possess an outer membrane As shown in the
  schematic drawings in Figure 1-3.
• Cells that retained the stain were called gram
  positive.
• The gram-positive cell surface has two major
  structures the
• cell wall and the cell membrane. The cell wall of
  gram-positive cells is composed of multiple
  layers of peptidoglycan, which is a linear
  polymer of alternating units of
  N-acetylglucosamine (NAG) and N-acetylmuramic
  acid (NAM).

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• A short peptide chain is attached to muramic
  acid.
• Gram-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 surface and may
  extend through the peptidoglycan layer to the
  outer surface.
• The peptidoglycan layer of a gram-negative cell
  is generally a single monolayer.
• An outer membrane surrounding the gram-negative
  cell is composed of 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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• Membranes-
• 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 its entire thickness.
• Some of these proteins provide structural support
  to the membrane while others function in the
  transport of sugars, amino acids, and other
  metabolites.
• The cytoplasmic membrane mediates not only these
• functions but also other important physiological
  activities.
• These include solute transport , oxidative
  phosphorylation
• through electron transport, photosynthetic
  electron
• transport in photosynthetic prokaryotes ,
  maintenance
• of electrochemical gradients and ATP 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 the rest of the membrane functions.
• Phospholipid forms both inner and outer leaflets
  of the cytoplasmic
• membrane, but the membrane is asymmetrical due to
  proteins present
• in 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. Though
• water can diffuse through the membrane, the
  diffusion rate is too low

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• Cytoplasm
• The 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-developed
• intracellular organelles such as mitochondria,
  chloroplasts and
• endoplasmic reticulum in addition to the nucleus.
  With only a few
• exceptions, prokaryotic cells do not have
  subcellular organelles
• within the cytoplasm.
• Prokaryotic cytoplasm contains DNA, ribosomes,
  proteins, RNA, salts and metabolites and is
  viscous due to the high concentration of
  macromolecules
• Some of these macromolecules form aggregates,
  while others are soluble. The soluble part is
  called the cytosol.
• Proteins in the cytoplasm are in high
  concentration and can interact with each other to
  form a kind of network.
• The enzymes involved in a particular metabolic
  process are adjacent for their required
  interaction.
• The term metabolon has been proposed to
  describe such a set of enzymes and their
  cofactors involved in such a fashion.

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• Capsules.
• 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 an amino acid).
• The capsule of Streptococcus pneumoniae type III
  is composed of glucose and glucuronic acid in
  alternating ß-1, 3- and ß-1, 4- linkages.
• This capsular polysaccharide, sometimes referred
  to as pneumococcal polysaccharide, is responsible
  for the virulence of the pneumococcus.
• Bacillus anthracis, the anthrax bacillus,
  produces a polypeptide capsule composed of
  D-glutamic acid subunits, which
• is a virulence factor for this organism.

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Capsules of Streptococcus pneumoniae
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• Flagella and pili
• Motile 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 cell surface structure and connected
  to the filament through the hook.
• The number and location of flagella vary
  depending on the bacterial
• species.
• 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 recognizing
• the appropriate receptor molecules.

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• The fimbria, also known as the pilus (plural,
  pili), is observed in
• many Gram-negative bacteria but rarely in
  Gram-positive bacteria.
• Fimbriae have been proposed as the fibrils that
  mediate attachment
• to surfaces. For this reason, the term pilus
  should be used only to
• describe the F-pilus, the structure that mediates
  conjugation.

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