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LECTURE 2:. Prokaryotic Cell Structure and Function. Microbiology and Virology; 3 Credit hours Atta- ur - Rahman School of Applied Biosciences (ASAB) National University of Sciences and Technology (NUST). Bacterial Cell Wall.

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Prokaryotic cell structure and function


Prokaryotic Cell Structure and Function

Microbiology and Virology; 3 Credit hours

Atta-ur-Rahman School of Applied Biosciences (ASAB)

National University of Sciences and Technology (NUST)

Bacterial cell wall
Bacterial Cell Wall

  • The cell wall is the layer, usually fairly rigid, that lies just outside the plasma membrane.

  • After Christian Gram developed the Gram stain in 1884, it soon became evident that most bacteria could be divided into two major groups based on their response to the Gram-stain procedure

    • Gram Positive Bacteria

    • Gram Negative Bacteria

Bacterial cell wall1
Bacterial Cell Wall

  • All the structures from the plasma membrane outward, the Cell envelope

  • This includes the plasma membrane, cell wall and structures like capsules if present.

  • Space between the plasma membrane and the outer membrane; Peri-plasmic space.

Peptidoglycan structure
Peptidoglycan Structure

  • Peptidoglycan (aka murein) is a mesh-like polymer composed of many identical subunits.

  • The polymer contains two sugar derivatives,

    • N-acetyl-glucosamine

    • N-acetyl-muramicacid

  • Several different amino acids

Prokaryotic cell structure and function

Peptidoglycan Structure

In order to make a strong, mesh-like polymer, chains of linked peptidoglycan subunits must be joined by cross-links between the peptides. Often the carboxyl group of the terminal D-alanine is connected directly to the amino group of di-aminopimelicacid,

but a peptide interbridge may be used instead. Most gram-negative cell wall peptidoglycan lacks the peptide interbridge. This cross-linking results in an enormous peptidogly- can sac that is actually one dense, interconnected network .

Peptidoglycan structure teichoic acids
Peptidoglycan Structure, Teichoic Acids

  • Gram positive cell walls usually contain large amounts of teichoic acids, polymers of glycerol or ribitol joined by phosphate groups

  • The teichoic acids are covalently connected to either the peptidoglycan itself or to plasma membrane lipids; in the latter case they are called lipo-teichoicacids.

  • They are negatively charged, help give the gram-positive cell wall its negative charge.

  • They are important in maintaining the structure of the wall.

  • Teichoicacids are not present in gram negative bacteria.

Teichoic acid structure
Teichoic Acid Structure

The segment of a teichoicacid made of phosphate, glycerol, and a side chain, R. R may represent D-alanine, glucose, or other molecules.

Peri plasmic space of gram positive bacteria
Peri-plasmicSpace of Gram-Positive Bacteria

  • The substance that occupies the periplasmic space is the periplasm.

  • The periplasm has relatively few proteins; this is probably because the peptidoglycan sac is porous and any proteins secreted by the cell usually pass through it.

  • Enzymes secreted by gram-positive bacteria are called exoenzymes.

  • Serve to degrade polymeric nutrients

Gram negative cell walls
Gram-Negative Cell Walls

  • Gram-negative cell walls are much more complex than gram-positive walls.

  • The periplasmic space of gram-negative bacteria is also strikingly different

  • It ranges in size from 1 nm to as great as 71 nm

  • It may constitute about 20 to 40% of the total cell volume

  • Nutrient acquisition—for example, hydrolytic enzymes

Gram negative cell walls1
Gram-Negative Cell Walls

  • Some periplasmic proteins are involved in energy conservation

  • The denitrifying bacteria, which convert nitrate to nitrogen gas, and bacteria that use inorganic molecules as energy sources (chemolithotrophs) have electron transport proteins in their periplasm.

  • Other periplasmicproteins are involved in peptidoglycan synthesis and the modification of toxic compounds that could harm the cell.

Lipopolysaccharides lpss
Lipopolysaccharides (LPSs)

  • Outer to the peptidoglycan layer is lipopolysaccharides (LPSs)

  • LPS contain both lipid and carbohydrate, and consist of three parts:

    • lipid A

    • the core polysaccharide

    • the O side chain

Prokaryotic cell structure and function

  • The lipid A region contains two glucosamine sugar derivatives, each with three fatty acids and phosphate or pyrophosphate attached.

  • The fatty acids attach the lipid A to the outer membrane, while the remainder of the LPS molecule projects from the surface.

  • The core polysaccharide is joined to lipid A.

Lipopolysaccharides lpss1
Lipopolysaccharides (LPSs)

  • Peptidoglycan layer and LPS are joined in two ways

  • The first is by Braun’s lipoprotein, the most abundant protein in the outer membrane.

  • This small lipoprotein is covalently joined to the underlying peptidoglycan, and is embedded in the outer membrane by its hydrophobic end.

  • The second linking mechanism involves the many adhesion sites joining the outer membrane and the plasma membrane.

Functions of lipopolysaccharides lps
Functions of Lipopolysaccharides (LPS)

  • The core polysaccharide usually contains charged sugars and phosphate, LPS contributes to the negative charge on the bacterial surface.

  • Bacterial attachment to surfaces and biofilm formation.

  • It aids in creating a permeability barrier to restrict the entry of bile salts, antibiotics, and other toxic substances that might kill or injure the bacterium.

  • The O side chain of LPS is also called the O antigen because it elicits an immune response.

  • G negative bacteria are able to rapidly change the antigenic nature of their O side chains, thus escaping host defenses.

Prokaryotic cell structure and function


Lipoteichoic acid

Peptidoglycan-teichoic acid

Cytoplasmic membrane





Outer Membrane

Braun lipoprotein

Periplasmic space

Inner (cytoplasmic) membrane


What happens in gram staining
What happens in Gram staining?

  • Crystal Violet (purple)

    • Primary stain; positive stain.

    • Stains all cell walls purple.

  • Iodine

    • Mordant (Any substance used to facilitate the fixing of a dye to a fibre).

    • Combines with CV to form an insoluble complex that gets trapped in thicker peptidoglycan layers.

  • Ethanol

    • Decolorizer.

    • CV complex washed out of Gram negative organisms because it cannot be trapped by outer layer.

  • Safranin (pink)

    • Counterstain.

    • Stains all cells, but only the negative ones actually appear pink.

Prokaryotic cell structure and function

G -ve

G +ve

Archaeal cell walls

  • Archaeal wall structure and chemistry differ from those of the Bacteria.

  • Archaeal cell walls lack peptidoglycan and also exhibit considerable variety in terms of their chemical make-up.

Methanobacteriumformicicum, and

(b)Thermoproteustenax. CW, cell wall; SL, surface layer; CM, cell membrane or plasma membrane; CPL, cytoplasm.

Gram positive type archae
Gram Positive Type Archae

  • Their wall chemistry varies from species to species but usually consists of complex heteropolysaccharides.

  • Methanobacterium and some other methane-generating archaea (methanogens) have walls containing pseudomurein

  • A peptidoglycan like polymer that has L-amino acids instead of D-amino acids in its cross-links

  • N-acetyltal-osaminuronicacid instead of N-acetylmuramic acid,

  • β(1→3) glycosidic bonds instead of β(1→4) glycosidic bonds.

The structure of pseudomurein
The Structure of Pseudomurein

Archaeaβ(1→3) Glycosidic Bond

Bacteria β(1→4) Glycosidic Bond

Gram negative type archae
Gram Negative Type Archae

  • Many archaea that stain gram negative have a layer of glyco-protein or protein outside their plasma membrane

  • Some methanogens (Methanolobus), salt-loving archaea (Halobacterium), and extreme thermophiles (Sulfolobus, Thermoproteus, and Pyrodictium) have glycoproteins in their walls.

  • The layer may be as thick as 20 to 40 nm.

  • In contrast, other methanogens (Methanococcus, Methanomicrobium, and Methanogenium) and the extreme thermophile Desulfurococcus have protein walls.