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Structure of Typical Bacterial Cell

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Structure of Typical Bacterial Cell Structure of typical Bacterial cell By: Dr. MohammedAzim Bagban External Structure to the cell wall • Capsule & slime layer • Surface layer • Surface appendages Capsule • The bacterial capsule is a very large structure of many bacteria. • It is a polysaccharide layer that lies outside the cell envelope, and is thus deemed part of the outer envelope of a bacterial cell. • It is a well-organized layer, not easily washed off, and it can be the cause of various diseases. Slime layer • A slime layer in bacteria is an easily removable (e.g. by centrifugation), unorganized layer of extracellular material that surrounds bacteria cells. • Specifically, this consists mostly of exopolysaccharides, glycoproteins, and glycolipids. • Therefore, the slime layer is considered as a form of glycocalyx. • The slime layer is not to be confused with the S-layer Capsule • The capsule is found most commonly among gram-negative bacteria: • Escherichia coli (in some strains) • Neisseria meningitidis • Klebsiella pneumoniae • Haemophilus influenzae • Pseudomonas aeruginosa • Salmonella spp. Capsule composition • some gram-positive bacteria may also have a capsule: • Bacillus megaterium for example, synthesizes a capsule composed of polypeptide and polysaccharides. • Streptococcus pyogenes synthesizes a hyaluronic acid capsule. • Streptococcus pneumoniae has at least 91 different capsular serotypes. These serotypes are the basis for the pneumococcal vaccines. • Streptococcus agalactiae produces a polysaccharide capsule of nine antigenic types that all contain sialic acid • Staphylococcus epidermidis • Staphylococcus aureus Functions of capsule • Significant water – protection against desiccation (extreme dryness). • Inhibits the movement of nutrients from cell. • Serve as source of nutrition in certain bacteria (e.g. Streptococcus mutans). • Stickiness helps in adherence on solid surfaces. • Protection against phagocytosis and contributes the virulent to the pathogens. • Function as antigen for their identification. • Due to charged substances they repel other cells in suspension and prevent bacterial cell aggregation and setteling Surface layer (S-Layer) • An S-layer (surface layer) is a part of the cell envelope found in almost all archaea, as well as in many types of bacteria. • It consists of a monomolecular layer composed of identical proteins or glycoproteins. Surface layer (S-Layer) • The terminology “S-layer” was used the first time in 1976. • The general use was accepted at the "First International Workshop on Crystalline Bacterial Cell Surface Layers, Vienna (Austria)" in 1984, and in the year 1987 S-layers were defined at the European Molecular Biology Organization Workshop on “Crystalline Bacterial Cell Surface Layers”, Vienna as “Two-dimensional arrays of proteinaceous subunits forming surface layers on prokaryotic cells” Surface layer (S-Layer) • Pattern resembles the floor tiles • Variety of symmetries like hexagonal, tetragonal, oblique, square etc. depending upon the structure of glycoprotein subunit. Function of Surface layer (S-Layer) • protection against bacteriophages, Bdellovibrios (parasitic), and phagocytosis • resistance against low pH • barrier against high-molecular-weight substances (e.g., lytic enzymes) • adhesion (for glycosylated S-layers) • stabilization of the membrane • resistance against electromagnetic stress (e.g. ionizing radiations and high temperatures) • provision of a periplasmic compartment in Gram-positive prokaryotes together with the peptidoglycan and the cytoplasmic membranes • anti-fouling properties. • molecular sieve and barrier function. Cell wall of bacteria • A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. • Bacterial cell walls are made of peptidoglycan (also called murein), which is made from polysaccharide chains cross-linked by unusual peptides containing D-amino acids. Bacterial cell wall Function: Shape and protection Structure: Distinguishes groups of bacteria Cells that Gram stain - Gram positive and Gram negative • Cells that are different - Genus Mycobacterium and Norcardia - Stained using Acid-fast staining techniques • Cells that lack cell walls – Will retain counterstain (second color applied during differential staining). Bacterial Cell Wall Peptidoglycan is a huge polymer of interlocking chains of alternating monomers. Provides rigid support while freely permeable to solutes. Backbone of peptidoglycan molecule composed of two amino sugar derivatives of glucose. The “glycan” part of peptidoglycan: - N-acetylglucosamine (NAG) - N-acetlymuramic acid (NAM) NAG / NAM strands are connected by interlocking peptide bridges. The “peptid” part of peptidoglycan. • Gram positive bacterial cell wall contains peptidoglycan and teichoic acid as major constitute. • Gram negative possess lipopolysachharide, peptidoglycan, lipoproteins and phospholipids. Peptidoglycan . Peptidoglycan (murein) is porous, elastic and stretchable. Complex polymer having repeating subunits . The sugar component consists of alternating residues of β-(1,4) linked N- acetylglucosamine (NAG) and N- acetylmuramic acid (NAM). Attached to the N-acetylmuramic acid is a peptide chain of three to five amino acids. The peptide chain can be cross-linked to the peptide chain of another strand forming the 3D mesh- like layer. • Diaminopimelic acid is an amino acid, representing an epsilon-carboxy derivative of lysine. • mDAP = L-Lysine Teichoic acid • Polymers of glycerol or ribitol joined by phosphodiester bonds. • Amino acids like D-ala or glucose are attached to ribitol (glycerol) groups. • They may linked with either peptidoglycan or lipids in plasma membrane respectively called as “Teichoic acid” or “Lipoteichoic acid”. Functions of teichoic acid • They are negatively charged so contribute to the negative charge of the cell surface. • Regulation of entry & exit of molecule • Prevention of cell lysis • Antigenicity • Attachment to the bacteriophages Gram negative cell wall • A thin peptidoglycan layer is present (This is much thicker in gram-positive bacteria). • Has outer membrane containing lipopolysaccharides (LPS, which consists of lipid A, core polysaccharide, and O antigen) in its outer leaflet and phospholipids in the inner leaflet. • Teichoic acids or lipoteichoic acids are absent Lipid A • Lipid A is a lipid component of an endotoxin held responsible for the toxicity of gram-negative bacteria. • Lipid A is not a glycerol lipid. • It consist of two glucosamine derivatives attaches to the three fatty acids and phosphates. Core polysaccharide • Oligosaccharide unit attached to the glocosamine residue of lipid-A. • Consists of keto- deoxyoctonate (KDO), seven carbon sugar (heptose), glucose, galactose, and NAG. O-polysaccharide (O-Ag) • The O antigen, consisting of many repeats of an oligosaccharide unit. • Sugars includes are : • -Galactose (Gal) • - Glucose (Glu) • - Rhamnose (Rha) • - Mannose (Man) Cell wall of Archaea • The cell wall of archaea is composed of S-layers and lack peptidoglycan molecules with the exception of methanobacteria who have pseudopeptidoglycan in their cell wall. Methanobacteria (archaea) Mycoplasma • Mycoplasma is a genus of bacteria that lack a cell wall around their cell membranes. • This characteristic makes them naturally resistant to antibiotics that target cell wall synthesis (like the beta-lactam antibiotics). • Mycoplasma species are the smallest bacterial cells yet discovered, can survive without Mycoplasma haemofelis oxygen, and come in various shapes. Other cell wall lacking bacteria • L-form bacteria, also known as Sam Cannon, L-phase variants, and cell wall-deficient (CWD) bacteria, are strains of bacteria that lack cell walls. • Two types of L-forms are distinguished: unstable L-forms, spheroplasts that are capable of dividing, but can revert to the original morphology, and stable L- forms, L-forms that are unable to Transmission electron revert to the original bacteria. micrograph of L-form Bacillus subtilis. • Bacterial morphology is determined by the cell wall. Since the L-form has no cell wall, its morphology is different from that of the strain of bacteria from which it is derived. • Typical L-form cells are spheres or spheroids. For example, L-forms of the rod-shaped bacterium Bacillus subtilis appear round when viewed by phase contrast microscopy or by transmission electron microscopy. • in a Gram stain test, the L-forms always colour Gram-negative, due to the lack of a cell wall. .
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