Chapter 2: Notes on Selected Antibiotics

Penicillins

Benzylpenicillin (Penicillin G)

• Narrow-spectrum; active against streptococci, meningococci, T. pallidum, some anaerobes.
• Ineffective against S. aureus due to penicillinase.
• Poor oral absorption – IV/IM only.
• Short half-life; frequent dosing or continuous infusion needed.

Phenoxymethylpenicillin (Penicillin V)

• Oral form of benzylpenicillin.
• Poor bioavailability.
• For mild streptococcal infections (e.g. pharyngitis).

Amoxicillin

• Broad-spectrum; effective against many Gram-positive and some Gram-negative organisms.
• Excellent oral absorption.
• Uses: otitis media, sinusitis, bronchitis, UTIs, typhoid, Helicobacter pylori.

Ampicillin

• Similar to amoxicillin.
• More used parenterally.
• Listeria monocytogenes, enterococci.

Beta-Lactam/Beta-Lactamase Inhibitor Combinations

Amoxicillin-clavulanate

• Clavulanate inhibits beta-lactamases.
• Effective against beta-lactamase-producing H. influenzae, M. catarrhalis, S. aureus.
• Uses: respiratory tract infections, skin/soft tissue infections, UTIs.

Piperacillin-tazobactam

• Broadest spectrum among penicillins.
• Active against Pseudomonas aeruginosa and anaerobes.
• Useful in hospital-acquired infections and intra-abdominal sepsis.

Cephalosporins

Note: None active against enterococci or Listeria.

1st Generation: Cefazolin, Cephalexin

• Good Gram-positive coverage (except MRSA).
• Limited Gram-negative activity.
• Uses: skin infections, surgical prophylaxis.

2nd Generation: Cefuroxime

• Increased Gram-negative activity.
• Uses: respiratory infections, UTIs.

3rd Generation: Cefotaxime, Ceftriaxone, Ceftazidime

• Excellent Gram-negative coverage.
• Ceftriaxone – once daily; good CSF penetration.
• Ceftazidime – active against P. aeruginosa.

4th Generation: Cefepime

• Broad-spectrum; good for febrile neutropenia, nosocomial infections.

Carbapenems

Imipenem, Meropenem, Ertapenem

• Broadest spectrum beta-lactams.
• Stable against most beta-lactamases.
• Reserve for serious nosocomial infections or ESBL organisms.
• Imipenem – risk of seizures; avoid in CNS disease.

Monobactams

Aztreonam

• Only active against aerobic Gram-negative bacteria.
• Safe in beta-lactam allergy (except ceftazidime).

Glycopeptides

Vancomycin

• Active against Gram-positive organisms, including MRSA.
• Poor oral absorption; oral use for C. difficile only.
• Monitor serum levels (AUC/MIC).
• Toxicity: nephrotoxicity, “red man” syndrome.

Aminoglycosides

Gentamicin, Amikacin, Tobramycin

• Bactericidal; concentration-dependent.
• Active against aerobic Gram-negatives.
• Toxicity: nephrotoxicity, ototoxicity.
• Monitor levels in long-term therapy or renal dysfunction.

Macrolides

Erythromycin, Azithromycin, Clarithromycin

• Good activity against Gram-positives, atypicals.
• Uses: community-acquired pneumonia, pertussis, chlamydia, H. pylori.
• Azithromycin – long half-life; fewer interactions.
• QT prolongation risk.

Tetracyclines

Doxycycline, Tetracycline

• Broad-spectrum: Gram-positive, Gram-negative, atypicals, malaria.
• Contraindicated in pregnancy and children <8 years.
• Photosensitivity, GI upset.

Fluoroquinolones

Ciprofloxacin, Levofloxacin, Moxifloxacin

• Broad-spectrum; good bioavailability.
• Ciprofloxacin – Gram-negative, including Pseudomonas.
• Moxifloxacin – respiratory pathogens, anaerobes.
• QT prolongation, tendonitis, C. difficile risk.

Lincosamides

Clindamycin

• Gram-positive and anaerobic coverage.
• Penicillin allergy alternative for skin/soft tissue infections.
• Risk of C. difficile diarrhoea.

Oxazolidinones

Linezolid

• Gram-positive coverage including MRSA and VRE.
• Oral and IV formulations.
• Bone marrow suppression, serotonin syndrome.

Sulfonamides and Trimethoprim

Cotrimoxazole (Trimethoprim-sulfamethoxazole)

• Broad-spectrum; active against Pneumocystis, Nocardia, some MRSA.
• Uses: UTIs, prophylaxis in immunosuppressed.
• Rash, bone marrow suppression, hyperkalaemia.

Nitroimidazoles

Metronidazole

• Anaerobes and protozoa.
• Drug of choice for C. difficile colitis.
• Disulfiram reaction with alcohol.

Others

Fosfomycin

• Oral; single dose for uncomplicated UTI.

Chloramphenicol

• Broad-spectrum.
• Risk of aplastic anaemia.

Tigecycline

• Glycylcycline; active against many MDR organisms.
• Not for bloodstream infections.

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