Microbial Polysaccharides
1. Cell Walls of Gram-Positive and GramNegative Bacteria
2. Peptidoglycan
3. Lysozyme
4. Penicillin
5. Lipopolysaccharides of Gram-Negative
Bacteria
Classification of Bacteria by GramStaining
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
Peptidoglycan
Structure
(in Staphylococcus
aureus only)
(in Staphylococcus aureus only)
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
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.
Penicillin
- discovered by Fleming in 1928.
- secreted by the mold Penicilium notatum.
- lysed many different bacteria.
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.
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.
Penicillin resistance:
Many pathogenic bacteria that are resistant
to penicillin produce a b-lactamase
(penicillinase).
Lipopolysaccharides (Endotoxins)
Drugs that are injected and medical
instruments must be “pyrogen-free.”
“Sterile” is not good enough.