Chemistry of Gram negative cell wall with reference to antibiotics

Contents

• Chemistry of cell wall

• Effects of antibiotics

Chemistry of gram negative cell wall

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 defenceagainst antibiotics.

Chemistry of cell wallTwo 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)

Composition of peptidoglycan polymer• Backbone is composed ofalternating N-acetylglucosamineand N-acetylmuramic acidresidues.

• A peptide chain of four alternatingD- and L-amino acids is connectedto the carboxyl group of N-acetylmuramic acid.

• In order to make a strong,meshlike polymer, chains of linkedpeptidoglycan subunits must bejoined by cross-links between thepeptides. Often the carboxyl groupof the terminal D-alanine isconnected directly to the aminogroup of diaminopimelic acid.

• Periplasmic proteins are involvedin peptidoglycan synthesis and themodification of toxic compoundsthat could harm the cell.

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.

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.

Effectsof

Antibiotics

Antibiotics:

• Antibiotic [anti, against, and bios, life]

• Microbial products or their derivatives that can kill susceptiblemicroorganisms or inhibit their growth.

• Special kind of chemotherapeutic agents usually obtained from livingorganisms.

There are total 4 antibiotics vulnerable to the gram negative cell wall.

• Penicillin

• Cephalosporin

• Vancomycin & Teicoplanin

• Cycloserine

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 peptidoglycansubunit

• 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

•Penicillin may stimulate special proteins called bacterial holins to form holesor lesions in the plasma membrane, leading directly to membrane leakage anddeath.

• Many penicillin-resistant bacteria produce penicillinase (also called β-lactamase), making penicillins unable to work.

• Other penicillin derivatives are made by physicians in which bacteria cannotdestroy β-lactame ring.

Attack site

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.

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.

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.

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