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

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

PeptidoglycanStructure

(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.

An acyl-enzyme intermediate is formed in the transpeptidation reaction.

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.

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.

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

Penicillin resistance:

Lipopolysaccharides (Endotoxins)

Drugs that are injected and medical instruments must be “pyrogen-free.”

“Sterile” is not good enough.