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Home , Bacterial capsule, Pseudopeptidoglycan, Slime layer

Structure of typical Bacterial

By: Dr. MohammedAzim Bagban External Structure to the

• Capsule & • Surface layer • Surface appendages Capsule

• The is a very large structure of many . • It is a 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 . • The slime layer is not to be confused with the S-layer Capsule • The capsule is found most commonly among gram-negative bacteria: • (in some strains) • • pneumoniae • • • spp.

Capsule composition

• some gram-positive bacteria may also have a capsule: • megaterium for example, synthesizes a capsule composed of polypeptide and . • synthesizes a capsule. • has at least 91 different capsular serotypes. These serotypes are the basis for the pneumococcal . • produces a polysaccharide capsule of nine antigenic types that all contain sialic acid • Staphylococcus epidermidis •

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. ). • Stickiness helps in adherence on solid surfaces. • Protection against 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 , 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 together with the 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 . • 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 and Norcardia - Stained using Acid-fast 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 as major constitute. • Gram negative possess lipopolysachharide, peptidoglycan, 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.

• is an , 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 “”.

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 (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 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.