Biology: Gram-Positive vs. Gram-Negative Bacteria

Gram-Positive vs. Gram-Negative Bacteria

• Gram-positive bacteria have a thick peptidoglycan layer, a cytoplasmic membrane, and lipopolysaccharide.
• Gram-negative bacteria have a thin peptidoglycan layer, a cytoplasmic membrane, and lipopolysaccharide.

Gram Staining

• Gram-positive bacteria: fixation, crystal violet, iodine treatment, decolorization, and counter stain safranin.
• Gram-negative bacteria: fixation, crystal violet, iodine treatment, decolorization, and counter stain safranin.

Acid-Fast Bacteria

• Mycobacterium has a peptidoglycan layer attached to an arabinogalactan polymer and surrounded by a wax-like lipid coat.
• Acid-fast staining interferes with phagocytosis.

Cell Wall Structure and Function

• The cell wall gives bacteria characteristic shape, protection, transport mechanisms, and aids pathogenicity.
• The cell wall provides advantages to humans, including classification and selective toxicity for anti-microbials.

Proteins

• Proteins form part of the cell wall.
• M protein of Group A Streptococcus protects from phagocytosis and is present as fine processes called fimbriae.
• Protein A of S. aureus binds Fc region of IgG and competes with neutrophil receptors for the Fc region of opsonizing antibodies.

Periplasmic Space

• Enzymes in the periplasmic space include proteases, phosphatases, lipases, nucleases, and carbohydrate degrading enzymes.
• Sugar transport systems and binding proteins are present in the periplasmic space.
• The periplasmic space contains components of the chemotaxis system, sensing the environment outside of the cell.

Outer Membrane

• The outer membrane is composed of lipopolysaccharide (LPS), also known as endotoxin.
• LPS is a powerful stimulator of the immune response, activating B cells, macrophages, and stimulating the release of IL1 and 6, tumor necrosis factor.
• LPS leads to fever, and if present in large amounts, septic shock and multi-organ failure.

Gram-Negative Cell Walls

• Gram-negative cell walls are thinner but more complex than Gram-positive cell walls.
• The cell wall contains a cytoplasmic membrane, a thin layer of peptidoglycan, and a periplasmic space with hydrolytic enzymes.

Outer Membrane

• The outer membrane is unique to Gram-negative prokaryotes and maintains structure and forms a permeability barrier to large molecules.
• The outer membrane provides protection from adverse environmental conditions and has a unique outer edge.

LPS

• LPS has three structural sections: lipid A, core polysaccharide, and O polysaccharide.
• Lipid A is anchored in the membrane and is responsible for endotoxic activity.
• Lipid A is very toxic to the host and is released in large quantities when cells die.

LPS

• The core polysaccharide is branched and contains 9-12 sugars, including the unusual sugar 2 keto-3-deoxy-octanate (KDO).
• The core region is constant for many species of bacteria.
• The O antigen is a long linear polysaccharide that distinguishes serotypes of bacterial species.

Membrane Pores

• Porin protein forms channels in the membrane, allowing transport of larger metabolites.
• The membrane is permeable to hydrophilic molecules, but less so to hydrophobic molecules.

Bacterial Cell Wall Components

• Cytoplasmic membrane contains ion pumps, synthetic enzymes, and a coiled portion called the mesosome.
• The mesosome acts as an anchor to bind and pull apart daughter chromosomes during cell division.

Peptidoglycan

• Peptidoglycan is cross-linked with a tetrapeptide and is characteristic for each specific bacterium.
• In Gram-positive bacteria, cross-linking involves a peptide interbridge.
• In S. aureus, each interbridge peptide consists of 5 molecules of the amino acid glycine connected by peptide bonds.

Peptidoglycan: Diversity

• Tetrapeptide contains both D and L amino acids.
• The first two amino acids may vary for different organisms.
• The third amino acid is essential for cross-linking.
• D-amino amino acids include lysine and diaminopimelic acid (DAP).

Additional Components

• In Gram-positive bacteria, peptidoglycan forms multiple layers and is often cross-linked in 3 dimensions.
• Teichoic acid is present in the wall and is an acidic polysaccharide that contains glycerophosphate or ribitol phosphate residues.

Peptidoglycan (Murein)

• Peptidoglycan is common to all types of bacterial cell walls.
• It is composed of a basic structure of two sugar derivatives connected in β1-4 linkages.
• The glycan chains are connected by peptide cross-links of four amino acids to give rigidity.

Peptidoglycan Specifics

• Peptidoglycan forms a mesh-like exoskeleton but is porous enough to allow diffusion of metabolites.
• It is essential for survival in hostile conditions.
• Peptidoglycan can interfere with phagocytosis, is mitogenic for lymphocytes, and has pyrogenic activity.
• The β1-4 bond can be degraded by lysozyme.

Peptidoglycan

• Precursor form of the peptide has an extra D-ala, which is released during the cross-linking step.
• Prefabricated subunits are assembled on the inside of the cell and brought to the surface by a large phospholipid called bactoprenol.

Teichoic Acid and Lipoteichoic Acid

• Glycerol-containing acids are bound to membrane lipids of Gram-positives and are called lipoteichoic acids.
• They mediate attachment to host cells and provide antigenic determinants.
• Sugars, choline, or D-alanine may be attached to the hydroxyls of the ribose or glycerol, determining serotype.