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Questions and Answers
What component is found in both Gram-positive and Gram-negative bacteria?
What component is found in both Gram-positive and Gram-negative bacteria?
Which step in Gram staining differentiates Gram-positive from Gram-negative bacteria?
Which step in Gram staining differentiates Gram-positive from Gram-negative bacteria?
Which of the following is a characteristic of acid-fast bacteria?
Which of the following is a characteristic of acid-fast bacteria?
Which of the following is not a function of a bacterial cell wall?
Which of the following is not a function of a bacterial cell wall?
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What role does the cell wall structure play in aiding pathogenicity?
What role does the cell wall structure play in aiding pathogenicity?
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What is the function of the mesosome in bacterial cell division?
What is the function of the mesosome in bacterial cell division?
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What kind of linkage connects the two sugar derivatives in peptidoglycan?
What kind of linkage connects the two sugar derivatives in peptidoglycan?
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Which amino acids are involved in Gram-positive bacteria's peptidoglycan interbridge in S. aureus?
Which amino acids are involved in Gram-positive bacteria's peptidoglycan interbridge in S. aureus?
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Which acid is present in the cell wall of Gram-positive bacteria?
Which acid is present in the cell wall of Gram-positive bacteria?
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What is the role of D-amino acids in peptidoglycan?
What is the role of D-amino acids in peptidoglycan?
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What type of bond in peptidoglycan can be degraded by lysozyme?
What type of bond in peptidoglycan can be degraded by lysozyme?
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What is the function of bactoprenol in peptidoglycan synthesis?
What is the function of bactoprenol in peptidoglycan synthesis?
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What provides antigenic determinants in Gram-positive bacteria?
What provides antigenic determinants in Gram-positive bacteria?
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What feature of M protein of Group A Streptococcus helps protect it from phagocytosis?
What feature of M protein of Group A Streptococcus helps protect it from phagocytosis?
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Which enzyme is not found in the periplasmic space?
Which enzyme is not found in the periplasmic space?
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What function does the outer membrane of Gram-negative prokaryotes serve?
What function does the outer membrane of Gram-negative prokaryotes serve?
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What is the main component leading to the endotoxic activity of LPS?
What is the main component leading to the endotoxic activity of LPS?
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Which of the following statements is true about the core polysaccharide of LPS?
Which of the following statements is true about the core polysaccharide of LPS?
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What is the main characteristic of the O antigen in LPS?
What is the main characteristic of the O antigen in LPS?
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Which of the following is a component of the periplasmic space's chemotaxis system?
Which of the following is a component of the periplasmic space's chemotaxis system?
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Which feature is unique to the outer membrane of Gram-negative prokaryotes?
Which feature is unique to the outer membrane of Gram-negative prokaryotes?
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Study Notes
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.
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Description
Learn about the differences between Gram-positive and Gram-negative bacteria, including their cell structure and Gram staining processes.
