Week 6 Pharmacology

Mechanisms of antimicrobial action

  • Lyse cell membrane
  • Inactivate microbe
  • Inhibit cell wall synthesis
  • Inhibit microbial ribosome
  • Inhibit enzymes required form nucleoside/DNA synthesis
  • Inhibit machinery of viral replication



Stop or slow bacterial growth, need immune system to finish off the microbe.



Results in bacterial death


Mechanisms of resistance

  • Decrease penetration of antibiotic – eg biofilm
  • Increase efflux with pumps
  • Alter site of action
  • Develop an enzyme to breakdown the antibiotic



  • Bacteriocidal
    • Beta lactam antibiotic
    • All have beta lactam ring that binds to D-ala- D ala of PBP and intefere with transpeptidation of bacterial cell wall
    • Cell lyse as a result
    • Only kills when the cells are growing and synthesising cell wall
  • Natural Penicillins e.g. Benzylpenicillin
    • GP org and GNC
    • No GN bacilli cover
    • Susceptible to beta lactamase
  • Semi-synthetic Penicillins e.g. Flucloxacillin, Dicloxacillin
    • Resistant to staph B lactamase
    • Effective against staph and strep
    • No effect on enterococci, anaerobic bacteria or GN Bacteria
  • Extended-spectrum Penicillins e.g. Amoxycillin / Ampicillin. Ticarcillin. Piperacillin.
    • Antipseudomonal
    • GP and Gn coverage
    • Susceptible to beta lactamase
  • Side Effects – allergy



Also beta lactam containing but less susceptible to beta lactamase

First Second Third Fourth




Ceftriaxone Cefepime
GP cocci, no pseudomonas cover Anaerobes, some GN, GP, no pseudomonas cover GP and expanded GN cover, cross BBB, effective against some b lactamase strains not all. Good pseudomonas cover, can be hydrolysed by extended spectrum B lactamase, penetrates well into csf
  • Renally cleared
  • SE – allergy, cross allergenicity with penicillins 5-10%, local irritation after injection, disulfiram like effect with some. Renal toxicity/nephritis/ATN.


  • Clavulanic acid is a beta lactamase inhibitor.
  • Carbapenams – eg meropenam – structurally related to B lactam. Good activity against GN rods, GP and anaerobes. Resistant to most Beta lactamases.



  • bacteriocidal
  • For gram positive bacteria esp staph (methicillin resistant staph)
  • Watersoluble, needs parenteral administration.
  • Inhibits cell wall synthesis by binding to D-ala D-Ala terminus and prevents further elongation at the site and therefore prevents peptidoglycan cross linking.
  • Oral vanc used for clostridium difficile
  • SE – phlebitis at site of injection, red man syndrome (flushing due to histamine release)



  • Bacteriostatic
  • GP, GN, anaerobes, chlamydia, mycoplasma, rickettsia
  • Enter partly by diffusion and partly by active transport. Bind reversibly to 30S subunit of ribosome preventing ongoing growth of the peptide.
  • Resistance is mediated by efflux pump, impaired entry, proteins intefering with binding at ribosome, enzymatic inactivation.
  • Doxycycline not renally excreted.
  • SE – crosses placenta, chelates with calcium so deposits in teeth and growing bones. Nausea, vomiting, diarrhoea, photosensitivity, renal toxicity.
  • Metabolised in liver and excreted in mainly bile and urine.



  • All contain macrocyclic ring.
  • Bacteriostatic, bacteriocidal at high concetration
  • Pneumonia, diptheria, chlamydial, ocular infections.
  • Good substitute for penicillin.
  • Bind to 50S ribosome and blocks aminoacyl translocation. Covers gram positive and negative organism.
  • Erythromycin. Roxithromycin. Clarithromycin Azithromycin
  • SE – annorexia, nausea, vomiting, diarrhoea, hepatic toxicity/hepatitis, inhibits cytP450 so results in drug interactions.



  • Bacteriostatic – aerobic and anaerobic GP and GN organisms.
  • Binds reversibly to bacterial 50S ribosome and inteferes with protein synthesis
  • Hepatic metabolism, renally excreted