The document summarizes the chemistry of the gram-negative cell wall and the effects of various antibiotics. It describes the general structure of the gram-negative cell wall, which has an outer membrane, peptidoglycan layer, and periplasmic space. The peptidoglycan is composed of sugars and amino acids that form a mesh-like polymer. The outer membrane contains lipopolysaccharides that protect the cell from host defenses. Several classes of antibiotics target aspects of cell wall synthesis, such as penicillins and cephalosporins which inhibit peptidoglycan cross-linking, and vancomycin which blocks transpeptidation.

1. Chemistry of
Gram negative
cell wall with
reference to
antibiotics

2. Contents
• Chemistry of cell wall
• Effects of antibiotics

3. Chemistry of gram
negative cell wall

4. Cell wall:
• Layer, usually fairly rigid, that lies just outside the
plasma membrane.
• Helps determine the shape of the cell
• Helps protect the cell from osmotic lysis
• Can protect the cell from toxic substances; and in
pathogens
General structure:
• It has a 2 to 7 nm peptidoglycan layer covered
by a 7 to 8 nm thick outer membrane.
• One important feature is a space that is frequently
seen between the plasma membrane and the outer
membrane called as periplasmic space.
(ranges in size from 1 nm to as great as 71 nm).
• The space filled with a metrial called as periplasm.
• The most outer membrane is also an important
constituent plays an important role in defence
against antibiotics.

5. Chemistry of cell wall
Two constituents are important to understand the basic
chemistry:
•Peptidoglycan
• Outer membrane
Peptidoglycan structure:
• Also known as murein, is an enormous mesh like polymer composed of many
identical subunits.
• Polymer contains:
oTwo sugar derivatives
 N-acetylglucosamine
 N-acetylmuramic acid
o Several different amino acids (D-glutamic acid, D-alanine, and
mesodiamino pimelic acid)

6. Composition of
peptidoglycan polymer
• Backbone is composed of
alternating N-acetylglucosamine
and N-acetylmuramic acid
residues.
• A peptide chain of four alternating
D- and L-amino acids is connected
to the carboxyl group of N-acetylmuramic
acid.
• In order to make a strong,
meshlike polymer, chains of linked
peptidoglycan subunits must be
joined by cross-links between the
peptides. Often the carboxyl group
of the terminal D-alanine is
connected directly to the amino
group of diaminopimelic acid.
• Periplasmic proteins are involved
in peptidoglycan synthesis and the
modification of toxic compounds
that could harm the cell.

7. Outer Membrane
• Lies outside the thin peptidoglycan layer and is linked to the cell in two ways.
 The first is by Braun’s lipoprotein, the most abundant protein in the outer
membrane.
 Covalently joined to the underlying peptidoglycan
 Embedded in the outer membrane by its hydrophobic end
 Involves many adhesion sites joining the outer membrane and the plasma
membrane.
• The two membranes appear to be in direct contact at these sites.
• The most unusual constituents of the outer membrane are its
lipopolysaccharides (LPSs):
These large, complex molecules contain both lipid and carbohydrate, and
consist of three parts:
 Lipid A
 The core polysaccharide
 The O side chain.

8. The lipid A region:
Contains two glucosamine sugar derivatives, each with three fatty acids and
phosphate or pyrophosphate attached.
• Fatty acids attach the lipid A to the outer membrane
• Remainder of the LPS molecule projects from the surface
The core polysaccharide is joined to lipid A
O side chain or O antigen:
Polysaccharide chain extending outward from the core.
Function of LPS:
• Protects pathogenic gram-negative bacteria from host defenses.
• O antigen elicits an immune response by producing some peculiar proteins.
• Many gram negative bacteria are able to rapidly change the antigenic nature of
their O side chains, thus thwarting host defenses.
• Lipid A portion of LPS often is toxic, hence forming endotoxins, inducing
septic shocks in their hosts.

9. Effects
of
Antibiotics

10. Antibiotics:
• Antibiotic [anti, against, and bios, life]
• Microbial products or their derivatives that can kill susceptible
microorganisms or inhibit their growth.
• Special kind of chemotherapeutic agents usually obtained from living
organisms.

11. There are total 4 antibiotics vulnerable to the gram negative cell
wall.
• Penicillin
• Cephalosporin
• Vancomycin & Teicoplanin
• Cycloserine

12. Penicillin
• Most are derivatives of 6-aminopenicillanic
acid and differ from one another with
respect to the side chain attached to its
amino group
• The most crucial feature of the molecule is
the β-lactam ring, which is essential for
bioactivity
Mechanism of action:
There are several proposed mechanisms:
• It is said that penicillins resemble the
terminal D-alanyl-D-alanine found on the
peptide side chain of the peptidoglycan
subunit
• They block the linkage between the
peptidoglycans (transpeptidation) thereby
blocking cross links, leading to osmotic
lysis.
• Penicillins may also bind to several
periplasmic proteins (penicillin-binding
proteins, or PBPs) and destroy bacteria by
activating their own autolytic enzymes. β-lactam ring

13. •Penicillin may stimulate special proteins called bacterial holins to form holes
or lesions in the plasma membrane, leading directly to membrane leakage and
death.
• Many penicillin-resistant bacteria produce penicillinase (also called β-
lactamase), making penicillins unable to work.
Attack site
• Other penicillin derivatives are made by physicians in which bacteria cannot
destroy β-lactame ring.

14. Cephalosporins
• Isolated from the fungus Cephalosporium.
• They contain a β-lactam structure that is very similar to that of the
penicillins, so they also inhibit transpeptidation.
• Given to those patients who are having penicillin allergy.
• Four generations are made of cephalosporins
 First generation is more effective against gram positive and less against
negative
 Second had improved effects against gram negative, with some anaerobe
range.
 Third generation was particularly developed against gram negative
bacteria, but some of them cross blood brain barrier and cause harm.
 Fourth generation is broad spectrum with excellent gram-positive and
gram-negative coverage. They, inhibit the growth of the difficult
opportunistic pathogen Pseudomonas aeruginosa.

15. Vancomycin and Teicoplanin
• Vancomycin is a glycopeptide antibiotic produced by Streptomyces
oreintalis.
• Blocks the transpeptidation reaction.
• Bactericidal for Staphylococcus and some members of the genera
Clostridium, Bacillus, Streptococcus, and Enterococcus.
• It is given both orally and intravenously.
• Particularly important in the treatment of antibiotic resistant staphylococcal
and enterococcal infections.
• Staphylococcus has become resistant to Vancomycin so, threatning health.
New generations of vancomycin must be made.
• Teicoplanin is a glycopeptide obtained from Actinoplanes teichomyceticus.
• Similar in structure and mechanism of action to vancomycin.
• Active against staphylococci, enterococci, streptococci, clostridia, Listeria,
and many other gram-positive pathogens.

16. Cycloserine
• Relatively simpler compound.
• Was originally discovered as an antibiotic produced by streptomycs and is
now manufactured through chemical synthesis.
• Main use is tuberculosis therapy.
• Due to potential undesirable side effects, its utilization is limited.
• Prevents the formation of peptide moeity
• Inhibits both alanine racemase and D-alanyl-D-alanine synthetase, the
enzymes involved in the synthesis of peptide side chains.

17. Questions
&
Answers