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Mechanism of action of antibiotics. The Sites of Activity in a Bacterial Cell for Various Antibiotics. Inhibition of bacterial cell wall. Cell Wall Biosynthesis. Stage I: Precursor formation Resulting in the production of the basic cell building block (UDP-acetylmuramyl-pentapeptide)

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cell wall biosynthesis
Cell Wall Biosynthesis

Stage I: Precursor formation

Resulting in the production of the basic cell building block (UDP-acetylmuramyl-pentapeptide)

Take place in the cytoplasm

Cycloserine inhibits the terminal reactions in this stage

cell wall biosynthesis1
Cell Wall Biosynthesis

Stage II: formation of a linear peptidoglycan

The precursor unit is carried from inside to outside

A number of modifications occur and the units are linked covalently to preexisting cell wall

Vancomycin and bacitracin act during this stage

cell wall biosynthesis2
Cell Wall Biosynthesis

Stage III: Crosslinking of the peptidoglycan

Take place entirely outside the cell membrane

The transpeptidase (the crosslinking enzyme) and other enzymes are inhibited by penicillins and cephalosporins

2 drugs that disrupt cell membrane function
2. Drugs That Disrupt Cell Membrane Function

A cell with a damaged membrane dies from a disruption in metabolism or lysis.

These drugs have specificity for a particular microbial group, based on differences in types of lipids in their cell membranes.

Polymyxins interact with phospholipids and cause leakage, particularly in gram-negative bacteria

  • Grammacidin forms bimolecular channel by association of two molecules of formyl end, forming hydrophilic pore through bacterial cell membrane leading to loose of H +, K + and water causing electrolyte imbalance and death
disruption of cell membrane
Disruption of cell membrane
  • Disruption of fungal membrane through binding with the sterol component of fungus cell membrane (Eucaryotic cell) causing a hydrophilic pores in the membrane leading to loss of H +, K + and water causing to electrolyte Imbalance and death e.g. polyene antibiotic as nystatin, amphotericin B
adverse effects of polymyxin
Adverse Effects of Polymyxin
  • Tachycardia
  • Eosinophilia
  • Fever
  • Nephrotoxicity and Neurotoxicity
  • Skin exanthemata
  • Urticaria

Nitrogen base, ribose, phosphate



Nucleosome (DNA + Histone)


Chromosome (1 centromere, 2 telomeres, several replication origins)

replication origins
Replication Origins

Bacterial chromosome has one, whereas each eukaryotic chromosome has many

Unique DNA segments contain multiple short repeated AT-rich sequences which recognized by multimeric origin-binding proteins that play a key role in assembling DNA polymerases and other replication enzymes at the sites where replication begins

dna replication
DNA Replication

Starts at replication origin

Helicase (unwinding of DNA) (replication fork)

single stranded DNA binding protein (SSB)

Leading and lagging strands

RNA Primase (placing of RNA primers)

dna replication1
DNA Replication
  • DNA Polymerase (the major work, 1000 nucleotides/sec)
  • RNA Primase (formation of Okazaki fragments)
  • Exonuclease (breakdown of RNA primers), and DNA polymerase replace them with DNA nucleotides
  • DNA Ligase (filling the gaps by inserting phosphate)

Inhibition of RNA synthesis (inhibition of transcription) through inhibition of RNA polymerase enzyme.

    • Antitumerantibiotics actinomycin, doxorubicin, and mithramycin bind to DNA inhibiting DNA templing and inhibiting RNA polymerase enzymes.
    • Rifamycins bind to RNA polymerase enzyme so inhibiting its activity
  • Inhibition of DNA synthesis by inhibition of either:
    • DNA gyrase enzyme thus inhibits DNA synthesis e.g. Novobiocin and nalidixic acid.
    • Covalently binds between DNA single strands, leading to inhibition of DNA synthesis e.g. Anti-tumer AB's as Bleomycin and Mitomycin.
protein synthesis
Protein Synthesis







Starts at promoter region (SIGMA = start)


Template (antisense) strand

Informational (coding, sense) strand

DNA-dependent RNA polymerase

RHO = stop

  • mRNA formation (immature) (pre-mRNA)
    • Methyl guanosine capping (5’) (helps ribosomes attach for translation)
    • Polyadenylation (up to 200 adenosine ribonucleotides) (3’) (helps transport the mRNA out of the nucleus and may stabilize the mRNA against degradation in the cytoplasm)
    • Splicing (spliceosomes)
    • Mature RNA formation


250-10,000 nucleotides


75-90 nuceotides, most abundant, up to 64 forms (20 amino acids)

  • rRNA
    • Ribosome (60% rRNA + 40% protein)
      • Svedberg factor (s)
      • Prokaryotes 70s (50s + 30s)
      • Eukaryotes 80s (60s + 40s)
      • P (peptidyl) and A (amino acyl tRNA) sites


Elongation (peptidase, translocase)