Pharmacology: Antibiotics

Question 1

What is the mechanism of beta-lactams and glycopeptides? Give examples

Answer 1

They are the most widely used.
Inhibit cell wall synthesis by binding to the proteins to prevent cross linking.
Examples of beta-lactams: penicillins, cephalosporins eg cetriaxone
Examples of glycopeptides: vancomycin

Question 2

What is the mechanism of tetracyclines, aminoglycosides and macrolides? Give examples

Answer 2

Inhibit protein synthesis
Examples of tetracyclines: doxycycline, tetracycline
Example of aminoglycosides: gentamicin
Example of macrolide: erythromycin, clarithromycin

Question 3

What is the mechanism of quinolones? Give examples

Answer 3

Inhibit nucleic acid synthesis by inhibiting DNA precursors

eg ciprofloxacin, trimethoprim

Question 4

What is the mechanism of polymixins?

Answer 4

Least common

Inhibit cell membrane function

Question 5

Name some circumstances where you might use prophylactic antibiotics

Answer 5

Peri-operative: only need to use small doses at the surgical site throughout surgery however there is no benefit after
Short term: 2-3 days for household meningitis contacts
Long term: asplenic pts, immunodeficiency eg HIV

Question 6

What factors should be taken into consideration when identifying the likely cause of an infection?

Answer 6

• duration eg gradual or sudden onset
• site eg focus or disseminated
• personal background eg immunosuppressed
• age eg different causes for different ages meningitis
• time of year eg flu in winter
• travel Hx
• occupation Hx
• PMH

Question 7

What factors should be considered when choosing which antibiotic to use?

Answer 7

1. Efficacy
2. Cost
3. Administration route eg GPs limited to oral
4. Patient safety ef allergies, toxicity, interactions, pregnant
5. Cost

Question 8

What adverse effects can result from antibiotics?

Answer 8

Pharmacological toxicities and drug interactions 
Allergic reactions (most commonly penicillins)
Impact on normal flora eg C diff infection

Question 9

Which antibiotics need therapeutic drug monitoring?

Answer 9

Gentamicin

Vancomycin (both toxic to the kidneys)

Question 10

What are the different ways bacteria can acquire resistance to antibiotics?

Answer 10

• inactivation of the antibiotic such as beta-lactamase
• alteration of the target site eg the penicillin binding protein
• active efflux of the antibiotic
• antibiotic can transfer resistance via gene transfer or chromosomal gene mutation