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TARGET AREAS OF MAJOR ANTIBIOTICS AND SYNTHETIC
AGENTS
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The microbial cell wall
Protein synthesis
Chromosome function and replication
Folate antagonists
The cytoplasmic membrane
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PEPTIDOGLYCAN
 Composed of sugar (glycan) chains cross - linked by short peptide
chains
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Contain alternating units of N - acetylmuramic acid and N –
acetylglucosamine
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Each N-acetylmuramic acid contains short peptide substituent
made up of four amino acids (stem peptides)
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Key feature contains D-isomer (D-alanine and D-glutamic acid) in
stem peptides and meso-diaminopimelic acid
VITAL ROLE OF PEPTIDOGLYCAN
 Responsible for maintenance of cell shape
 Gives mechanical strength of the bacterial cell
If damaged:
1. Bacterial cell becomes distorted
2. Cells swell and lyse resulting from high osmotic pressure
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Interferes with the early stage of synthesis
of peptidoglycan involving the assembly of
the dipeptide d-alanyl - d-alanine
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Inhibits racemase and ligase activity by
mimicking and binding to pyridoxal
phosphate (cofactor of enzymes)
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Also blocks the coupling of dipeptide to
three other amino acids forming he stem
peptide on UDP-N-acetylmuramic acid
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The
glycopeptides
vancomycin
and
teicoplanin act at the stage where the
peptidoglycan precursors are inserted into
the cell wall by the transglycosylase enzyme
on the outer face of the cell membrane
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Glycopeptides block this processes of
transglycosylase by binding to the
disaccharide
peptidoglycan
precursor,
specifically to the d-alanyl - d-alanine
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Resistance develop results from alteration in
D-alanyl-D-alanine substrate to D-alanyl-Dlactacte (seen in VRE)
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Glycopeptides must cross the cell wall to reach outer face of
cell membrane where transglycosylation takes place.
However, they are too large to penetrate the outer
membrane of gram-negative bacteria
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The β - lactam antibiotics inhibit transpeptidases by acting
as alternative substrates for the enzyme
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The vital cross - linking of the peptidoglycan is therefore
blocked but other aspect of cell growth occurs
Also activates autolysins
on cell wall
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Use in combination with susceptible β – lactams protecting
them from inactivation by the β – lactamases
Inhibitors are hydrolysed by the β - lactamases in the same
manner as susceptible β – lactam antibiotics. But instead
of rapid release from active site, it remains bound and
undergo one of several fates
Ex. It interact with second enzyme residue with active site of Blactamase forming irreversibly inhibited complex
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MYCOLIC ACID COMPONENTS
1. Arabinogalactan polysaccharide
2. Peptidoglycan
3. High-molecular weight lipids
a. mycolic acids
b. glycolipids
c. phospholipids
d. waxes
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Isoniazid interferes with mycolic acid synthesis by inhibiting
an enoyl reductase (InhA)
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It is converted inside the mycobacteria to a free radical
species by a catalase peroxidase enzyme, KatG. The active
free radicals then attack and inhibit the enoyl reductase,
InhA
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Ethambutol blocks assembly of the arabinogalactan
polysaccharide by inhibition of an arabinotransferase
enzyme
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They interfere with the synthesis of the β-1,3-d-glucan
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Without the glucan polymer, the integrity of the fungal cell
wall is compromised, yeast cells lose their rigidity and
become like protoplasts
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Effect is pronounced in Candida and Aspergillus species
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The process of protein synthesis involves the ribosome,
mRNA, a series of amino - acyltransfer RNA (tRNA)
molecules and accessory protein factors involved in
initiation, elongation and termination
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Bacterial ribosomes are smaller than their mammalian
counterparts. They consist of one 30S and one 50S subunit
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Most of the information on the mechanisms of action of
aminoglycoside – aminocyclitol (AGAC) antibiotics comes
from studies with streptomycin
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To interfere with the initiation and assembly of the bacterial
ribosome
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Streptomycin binds tightly to one of the protein components
of the 30S subunit which prevents initiation and assembly
of the ribosome
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1.
2.
Streptomycin exerts two effects:
inhibition of protein synthesis by freezing the initiation
complex
misreading of the codons through distortion of the 30S
subunit
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Binding of just one molecule of tetracycline to the bacterial 30S
subunit
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The effect is to block the binding of aminoacyl - tRNA to the A site
of the ribosome and halt protein synthesis
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Tetracyclines are bacteriostatic rather than bactericidal
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Resistance to the tetracyclines occurs through failure of the
active uptake system or the action of active efflux pumps, which
remove the drug from the cells before it can interfere with
ribosome function
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Of the four possible optical isomers of chloramphenicol, only Dthreo form is active
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This antibiotic selectively inhibits protein synthesis in bacterial
ribosomes by binding to the 50S subunit
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Chloramphenicol has a broad spectrum of activity, which covers
Gram - positive and Gram - negative bacteria, mycoplasmas,
rickettsia and chlamydia
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Has valuable property of penetrating into mammalian cells and is
therefore the drug of choice for treatment of intracellular
pathogens
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Erythromycin is a member of the macrolide group of antibiotics
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It selectively inhibits protein synthesis in a broad range of
bacteria by binding to the 50S subunit and blocks translocation
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By blocking the translocation process, erythromycin causes
release of incomplete polypeptides from the ribosome
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The azalides has same mechanism of actions with macrolides
but have improved intracellular penetration over the macrolides
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This agent binds selectively to a region of the 50S
ribosomal subunit close to that of chloramphenicol and
erythromycin.
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It blocks elongation of the peptide chain by inhibition of
peptidyl transferase
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Have been used in combination (in a 30 : 70 ratio) to treat
infections caused by staphylococci and enterococci,
particularly methicillin - resistant Staph. aureus (MRSA) and
VRE.
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Their action is synergistic
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The main target is the bacterial 50S ribosome: dalfopristin
alters the shape of the ribosome so that more quinupristin
can bind
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Oxazolidinones such as linezolid act at the early stage of
protein synthesis, preventing the formation of the initiation
complex between the 30S subunit, mRNA and fmet – tRNA
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The target of mupirocin is one of a group of enzymes which
couple amino acids to their respective tRNAs for delivery to
the ribosome and incorporation into protein
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The particular enzyme inhibited by mupirocin is involved in
producing isoleucyl – tRNA
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This steroidal antibiotic does not act on the ribosome itself,
but on one of the associated elongation factors, EF – G
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Binds with EF-G:GDP complex to the ribosome after one
round of translocation has taken place
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These agents bind to the component of the 50S bacterial
ribosome and block peptide bond formation by interfering
with the binding of the peptidyl transferase region with the
aminoacyl - tRNA substrates in the A and P sites on the
ribosome
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During replication the circular double helix must be
unwound to allow the DNA polymerase enzymes to
synthesize new complementary strands
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In a bacterium such as E. coli four different topoisomerase
enzymes are responsible for maintaining the shape of DNA
during cell division
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They act by cutting one or both of the DNA strands; they
remove and generate supercoiling, then reseal the strands
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The fluoroquinolones selectively inhibit topoisomerases II
and IV
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These agents also cause DNA strand breakage, but by a direct
chemical action rather than by inhibition of a topoisomerase
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The nitro group of metronidazole is converted to a nitronate
radical by the low redox potential within cells.
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The pyruvate:ferredoxin oxidoreductase (POR) is a major
metabolic pathway in anaerobic bacteria and protozoa used for
generation of ATP.
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This system converts metronidazole to its active form which then
attacks the DNA, producing strand breakage
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Binding of just one molecule of rifampicin inhibits the
initiation stage of transcription
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The action of rifampicin is specific for the microbial RNA
polymerase
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Rifabutin, which has enhanced activity against
Mycobacterium avium complex, is thought to act in the
same way as rifampicin
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This antifungal agent inhibits DNA synthesis at the early
stages involving production of the nucleotide thymidylic acid
(TMP)
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An important antifungal agent in the treatment of life threatening infections
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Resistance can occur due to active efflux of the drug from
the cells before it can inhibit DNA synthesis
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Bacterial and protozoal cells must synthesize dihydrofolic acid (DHF)
from pteridine, p - aminobenzoic acid (PABA) and glutamate.
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DHF is converted to tetrahydrofolic acid (THF) by the enzyme
dihydrofolate reductase (DHFR).
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THF supplies single carbon units for various pathways including DNA,
RNA and methionine synthesis
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Sulphonamides - they competitively inhibit the incorporation
of PABA
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Trimethoprim - selective inhibitor of bacterial DHFR
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Pyrimethamine - selective inhibitor of plasmodial DHFR
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Polymyxin E (colistin) is used in the treatment of serious
Gram - negative bacterial infections
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It binds tightly to the lipid A component of LPS in the outer
membrane of Gram - negative bacteria
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Permeability barrier is destroyed
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The polymyxin molecules can then penetrate to the
cytoplasmic membrane where they bind to phospholipids,
disrupt membrane integrity, and cause irreversible leakage
of cytoplasmic components
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The compound inserts itself into the cytoplasmic membrane
and drug molecules aggregate together forming channels
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The leakage of potassium ions from the cells results in
inhibition of macromolecular synthesis and cell death
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Amphotericin B and nystatin are the most commonly used
members of this group of antifungal agents
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Derive their action from their strong affinity towards sterols,
particularly ergosterol
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Polyene is pulled into the membrane interior, destabilizing
the structure and causing leakage of cytoplasmic
constituents
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The azole antifungal drugs act by inhibiting the synthesis of
the sterol components of the fungal membrane
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Cause rapid defects in fungal membrane integrity due to
reduced levels of ergosterol
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Inhibits the enzyme squalene epoxidase at an early stage in
fungal sterol biosynthesis
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Innate resistance
Phenotypic - resulting from adaptation to growth within a
specific environment
Conjugation
Mutations
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Inactivation of the drug
Alteration of the target
Reduced cellular uptake
Increased efflux