The library's textbooks are specialty-focused (surgery, orthopaedics, etc.) rather than pharmacology. I have sufficient foundational knowledge to provide a comprehensive, accurate answer on antibiotics. Let me now compile the full response.
Antibiotics: Classification, Dose, Indications & Contraindications
What Are Antibiotics?
Antibiotics are chemical substances derived from microorganisms or synthesized artificially that kill or inhibit the growth of bacteria. They act by targeting structures or processes unique to bacteria (cell wall, protein synthesis, DNA replication, cell membrane) without harming host cells.
Classification of Antibiotics
Antibiotics are classified by several overlapping systems:
A. By Mechanism of Action
| Mechanism | Drug Classes |
|---|
| Cell wall synthesis inhibition | Beta-lactams (penicillins, cephalosporins, carbapenems, monobactams), Glycopeptides (vancomycin) |
| Protein synthesis inhibition (30S ribosome) | Aminoglycosides, Tetracyclines |
| Protein synthesis inhibition (50S ribosome) | Macrolides, Chloramphenicol, Clindamycin, Linezolid |
| DNA/RNA synthesis inhibition | Fluoroquinolones, Rifamycins, Metronidazole |
| Cell membrane disruption | Polymyxins (colistin), Daptomycin |
| Folate synthesis inhibition | Sulfonamides, Trimethoprim |
B. By Chemical Structure
1. Beta-Lactams
(a) Penicillins
| Drug | Dose | Indications | Contraindications |
|---|
| Benzylpenicillin (Pen G) | 1-4 MU IV every 4-6 h | Streptococcal infections, meningococcal meningitis, syphilis, tetanus, gas gangrene | Penicillin allergy, severe renal failure (adjust dose) |
| Amoxicillin | 250-500 mg PO TDS; 1 g IV TDS | URTI, otitis media, sinusitis, UTI, H. pylori (triple therapy) | Penicillin hypersensitivity, infectious mononucleosis (causes rash) |
| Amoxicillin-Clavulanate (Co-amoxiclav) | 625 mg PO TDS; 1.2 g IV TDS | Beta-lactamase-producing organisms, LRTI, skin/soft tissue, bite wounds | Penicillin allergy, cholestatic jaundice history from prior use |
| Cloxacillin / Flucloxacillin | 250-500 mg PO QID; 1-2 g IV 6-hourly | MSSA infections: cellulitis, wound infections, osteomyelitis | Penicillin allergy; not for MRSA |
| Piperacillin-Tazobactam | 4.5 g IV every 6-8 h | Hospital-acquired infections, Pseudomonas, intra-abdominal, febrile neutropenia | Penicillin allergy |
(b) Cephalosporins
| Generation | Example Drugs | Dose | Indications | Contraindications |
|---|
| 1st Gen | Cephalexin, Cefazolin | Cephalexin 500 mg PO QID; Cefazolin 1-2 g IV 8-hourly | MSSA, strep skin/soft tissue, surgical prophylaxis | Cephalosporin allergy; caution in penicillin allergy (~2% cross-reactivity) |
| 2nd Gen | Cefuroxime, Cefaclor | Cefuroxime 750 mg IV 8-hourly | LRTI, sinusitis, otitis media, mixed aerobic-anaerobic | Same as above |
| 3rd Gen | Ceftriaxone, Cefotaxime, Ceftazidime | Ceftriaxone 1-2 g IV OD; Ceftazidime 1-2 g IV 8-hourly | Meningitis, gonorrhoea, gram-negative sepsis, typhoid | Same; Ceftriaxone avoided in neonates with hyperbilirubinemia |
| 4th Gen | Cefepime | 1-2 g IV 12-hourly | Febrile neutropenia, Pseudomonas, nosocomial infections | Cephalosporin allergy |
| 5th Gen | Ceftaroline | 600 mg IV 12-hourly | MRSA, CAP, skin/soft tissue (MRSA-active) | Cephalosporin allergy |
(c) Carbapenems
| Drug | Dose | Indications | Contraindications |
|---|
| Meropenem | 0.5-1 g IV 8-hourly (2 g for meningitis) | Multi-drug resistant gram-negatives, complicated intra-abdominal, nosocomial pneumonia | Beta-lactam allergy, seizure history (relatively) |
| Imipenem-Cilastatin | 500 mg IV 6-hourly | Broad-spectrum nosocomial infections | Seizure disorders, beta-lactam allergy |
| Ertapenem | 1 g IV/IM OD | Community-acquired infections, ESBL organisms | Beta-lactam allergy |
(d) Monobactams
| Drug | Dose | Indications | Contraindications |
|---|
| Aztreonam | 1-2 g IV 8-hourly | Gram-negative infections only (including Pseudomonas); useful in penicillin allergy | Aztreonam hypersensitivity; no gram-positive or anaerobe coverage |
2. Glycopeptides
| Drug | Dose | Indications | Contraindications |
|---|
| Vancomycin | 15-20 mg/kg IV 8-12 hourly (target trough 15-20 mg/L); 125 mg PO QID for C. diff | MRSA infections, endocarditis, C. difficile (oral only), gram-positive organisms in penicillin-allergic patients | Hypersensitivity; rapid IV infusion causes "Red Man Syndrome" (slow the rate); caution in renal failure |
| Teicoplanin | 400-800 mg IV/IM OD (after loading) | Similar to vancomycin | Glycopeptide allergy |
3. Aminoglycosides
| Drug | Dose | Indications | Contraindications |
|---|
| Gentamicin | 5-7 mg/kg IV OD (extended-interval); 3-5 mg/kg/day divided | Gram-negative sepsis, endocarditis (synergy), UTI, peritonitis | Pregnancy (ototoxicity), myasthenia gravis, renal failure (adjust), avoid with loop diuretics |
| Amikacin | 15-20 mg/kg IV OD | MDR gram-negative infections, MDR-TB | Same as gentamicin |
| Tobramycin | 5-7 mg/kg IV OD | Pseudomonas aeruginosa, gram-negative infections | Same as gentamicin |
| Streptomycin | 15 mg/kg IM OD | Tuberculosis (2nd line), brucellosis, plague | Pregnancy, renal failure, auditory/vestibular dysfunction |
Note: All aminoglycosides require therapeutic drug monitoring (TDM); they are nephrotoxic and ototoxic.
4. Macrolides
| Drug | Dose | Indications | Contraindications |
|---|
| Azithromycin | 500 mg PO OD x 3 days or 500 mg day 1 then 250 mg x 4 days | Atypical pneumonia (Mycoplasma, Chlamydophila, Legionella), URTI, STIs (Chlamydia), enteric fever, MAC prophylaxis | QT prolongation, hepatic failure, macrolide allergy |
| Clarithromycin | 500 mg PO/IV BD | H. pylori (triple therapy), CAP, LRTI, NTM infections | QT prolongation, severe hepatic impairment, statins/ergotamine (CYP3A4 interactions) |
| Erythromycin | 250-500 mg PO QID | CAP, URTI, Legionella, Campylobacter, rheumatic fever prophylaxis (penicillin allergy) | QT prolongation, hepatic disease, multiple drug interactions |
5. Tetracyclines
| Drug | Dose | Indications | Contraindications |
|---|
| Doxycycline | 100 mg PO/IV BD (loading: 200 mg once) | Atypical pneumonia, chlamydia, pelvic inflammatory disease, malaria prophylaxis, brucellosis, rickettsia, Lyme disease, acne | Pregnancy, children <8 years (tooth discolouration, bone growth inhibition), severe hepatic failure |
| Tetracycline | 250-500 mg PO QID | Acne, H. pylori (quadruple therapy), chlamydia | Same as doxycycline; also avoided with antacids/milk (chelation) |
| Minocycline | 100 mg PO BD | Acne, MRSA (adjunct), chlamydia | Same as above; vestibular side effects |
6. Fluoroquinolones (Quinolones)
| Drug | Dose | Indications | Contraindications |
|---|
| Ciprofloxacin | 500-750 mg PO BD; 400 mg IV BD | Gram-negative UTI, typhoid, Pseudomonas, GI infections (Salmonella, Campylobacter), anthrax | Children (cartilage toxicity - generally avoided), pregnancy, QT prolongation, myasthenia gravis, concomitant NSAIDs (seizure risk) |
| Levofloxacin | 500 mg PO/IV OD | CAP, complicated UTI, sinusitis, atypical pathogens | Same as ciprofloxacin; QT prolongation |
| Moxifloxacin | 400 mg PO/IV OD | CAP, skin/soft tissue, intra-abdominal; NO urinary use (poor urinary excretion) | QT prolongation, hepatic failure; NOT for UTI |
| Norfloxacin | 400 mg PO BD | UTI, gonorrhoea, traveller's diarrhoea | Same class effects; poor tissue penetration |
Class warning: All fluoroquinolones carry an FDA/EMA black box warning for tendinopathy/tendon rupture, peripheral neuropathy, and CNS effects. Use with caution in elderly and those on corticosteroids.
7. Sulfonamides & Trimethoprim
| Drug | Dose | Indications | Contraindications |
|---|
| Co-trimoxazole (TMP-SMX) | 160/800 mg PO BD; IV for severe infections; 1 DS tablet PO OD for PCP prophylaxis | UTI, PCP (Pneumocystis), toxoplasmosis prophylaxis, Stenotrophomonas, MRSA (adjunct), nocardia | G6PD deficiency, sulfonamide allergy, pregnancy (3rd trimester - kernicterus), severe renal/hepatic failure, neonates |
| Trimethoprim alone | 100-200 mg PO BD | Uncomplicated UTI | Folate deficiency, pregnancy, renal failure |
8. Nitroimidazoles
| Drug | Dose | Indications | Contraindications |
|---|
| Metronidazole | 400-500 mg PO TDS; 500 mg IV 8-hourly; 2 g PO stat (STI) | Anaerobic infections, C. difficile, H. pylori, bacterial vaginosis, trichomoniasis, intra-abdominal sepsis, Giardia, amoebiasis | 1st trimester pregnancy, alcohol (disulfiram-like reaction), severe hepatic impairment |
9. Chloramphenicol
| Drug | Dose | Indications | Contraindications |
|---|
| Chloramphenicol | 500 mg PO/IV QID; topical drops for eye infections | Typhoid (2nd line), meningitis (penicillin allergy), rickettsial infections, bacterial conjunctivitis (topical) | Neonates (Grey Baby Syndrome - immature glucuronidation), pregnancy, bone marrow suppression, prior aplastic anaemia |
10. Lincosamides
| Drug | Dose | Indications | Contraindications |
|---|
| Clindamycin | 150-450 mg PO QID; 600-900 mg IV 8-hourly | Anaerobic infections, MSSA/MRSA (some strains), dental infections, pelvic infections, toxin suppression in necrotising fasciitis, malaria (with quinine) | C. difficile colitis risk (high risk agent), clindamycin hypersensitivity |
11. Oxazolidinones
| Drug | Dose | Indications | Contraindications |
|---|
| Linezolid | 600 mg PO/IV BD | MRSA pneumonia, VRE, resistant gram-positive infections; excellent oral bioavailability (IV=PO) | MAOIs (serotonin syndrome), SSRIs, prolonged use causes myelosuppression and optic neuritis; avoid foods high in tyramine |
12. Polymyxins
| Drug | Dose | Indications | Contraindications |
|---|
| Colistin (Polymyxin E) | Dose individualized by body weight and renal function (e.g., 2.5-5 mg/kg/day IV in 2-4 divided doses) | Last-resort treatment for carbapenem-resistant Acinetobacter, Pseudomonas, Klebsiella (CRKP) | Renal failure (dose-limit nephrotoxicity); myasthenia gravis; neuromuscular blockade risk |
13. Anti-MRSA Agents
| Drug | Dose | Indications | Contraindications |
|---|
| Daptomycin | 4-6 mg/kg IV OD (8-10 mg/kg for bacteraemia/endocarditis) | MRSA bacteraemia, endocarditis, skin/soft tissue - NOT for pneumonia (inactivated by surfactant) | Pulmonary infections, myopathy (CK monitoring required), concomitant statins |
| Tedizolid | 200 mg PO/IV OD | MRSA skin/soft tissue infections | Similar class effects to linezolid |
14. Anti-TB Drugs (First-Line)
| Drug | Dose | Indications | Contraindications |
|---|
| Isoniazid (INH) | 5 mg/kg (max 300 mg) PO OD | Tuberculosis (all forms), latent TB prophylaxis | Hepatic disease, prior INH-induced hepatitis, pyridoxine deficiency |
| Rifampicin | 10 mg/kg (max 600 mg) PO OD | TB, leprosy, meningococcal prophylaxis, Staph carrier | Severe hepatic disease, porphyria; major CYP450 inducer (many drug interactions) |
| Pyrazinamide | 25 mg/kg PO OD | Tuberculosis (first 2 months) | Gout, severe hepatic disease |
| Ethambutol | 15-25 mg/kg PO OD | Tuberculosis (first 2 months) | Optic neuritis, children too young for visual testing |
Summary: Bactericidal vs. Bacteriostatic
| Bactericidal | Bacteriostatic |
|---|
| Beta-lactams, Aminoglycosides, Fluoroquinolones, Vancomycin, Metronidazole, Rifampicin, Daptomycin, Colistin | Tetracyclines, Macrolides, Sulfonamides, Clindamycin, Chloramphenicol, Linezolid |
In immunocompromised patients and endocarditis, bactericidal agents are preferred.
General Contraindications Across All Antibiotics
- Known hypersensitivity to the specific drug or drug class
- Inappropriate spectrum (e.g., using antibiotics for viral infections)
- No confirmed or suspected bacterial infection (stewardship principle)
- Certain agents avoided in pregnancy (tetracyclines, fluoroquinolones, aminoglycosides, sulfonamides in 3rd trimester, metronidazole in 1st trimester)
- Renal/hepatic impairment requires dose adjustment for renally/hepatically cleared drugs
- G6PD deficiency: avoid sulfonamides, chloramphenicol, nitrofurantoin
- Neonates: avoid chloramphenicol (Grey Baby Syndrome), avoid ceftriaxone with hyperbilirubinemia
This overview covers the major antibiotic classes. Clinical decisions should always account for local antibiograms, patient-specific factors (allergies, organ function, pregnancy), culture/sensitivity results, and antimicrobial stewardship principles.
