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Microbial Polysaccharides

Microbial Polysaccharides. 1. Cell Walls of Gram-Positive and Gram-Negative Bacteria 2. Peptidoglycan 3. Lysozyme 4. Penicillin 5. Lipopolysaccharides of Gram-Negative Bacteria . Classification of Bacteria by Gram-Staining.

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Microbial Polysaccharides

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  1. Microbial Polysaccharides • 1. Cell Walls of Gram-Positive and Gram-Negative Bacteria • 2. Peptidoglycan • 3. Lysozyme • 4. Penicillin • 5. Lipopolysaccharides of Gram-Negative Bacteria

  2. Classification of Bacteria by Gram-Staining Christian Gram, Danish physician about 120 years ago. Gram-positive bacteria retain the stain (crystal violet) and appear blue, whereas gram-negative bacteria usually appear pink due to the counterstain safranin. Crystal Violet Streptococci & E. coli

  3. Peptidoglycan Structure (in Staphylococcus aureus only)

  4. (in Staphylococcus aureus only)

  5. Lysozyme • discovered by Fleming in 1922. • present in tears, saliva, nasal secretions, etc., and also present in egg white. • lyses the cell walls of many bacteria Alexander Fleming

  6. Penicillin • discovered by Fleming in 1928. • secreted by the mold Penicilium notatum. • lysed many different bacteria. In the naturally occuring penicillin G (benzyl penicillin), R is a benzyl group. In the semi-synthetic penicillin derivative ampicillin, R is an aminobenzyl group.

  7. Penicillin • discovered by Fleming in 1928. • secreted by the mold Penicilium notatum. • lysed many different bacteria.

  8. Penicillin functions by specifically interfering with the cross-linking of peptidoglycan chains, a process that is catalyzed by glycopeptide transpeptidase. In S. aureus, the transpeptidase reaction involves an attack of the amino group of the pentaglycine bridge on the peptide bond between two D-ala residues to form a cross-link. An acyl-enzyme intermediate is formed in the transpeptidation reaction.

  9. Penicillin mimics the D-ala-D-ala moiety of the normal substrate and also forms an acyl-enzyme complex. However, the penicilloyl-enzyme complex is very stable. The result is that one penicillin molecule permanently inactivates one enzyme molecule.

  10. Penicillin resistance: Many pathogenic bacteria that are resistant to penicillin produce a b-lactamase (penicillinase).

  11. Lipopolysaccharides (Endotoxins)

  12. Drugs that are injected and medical instruments must be “pyrogen-free.” “Sterile” is not good enough.

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