Antimicrobial agents:

Properties of ideal antimicrobial agent:

• Should have powerful action against microorganism
• It should be specific in action i.e. acts specifically on invading organisms without any toxicity to host.
• Should not be inactivated rapidly by tissue enzyme or G.I microflora
• Should have good oral bioavailability and penetrate efficiently to various body tissues and fluids.
• Should have long elimination half-life and not rapidly excreted by kidneys/bile
• Should not favour bacterial resistance and show cross-resistance with other antimicrobial agents.
• Should not interfere with host immune mechanisms
• Should be non-allergic
• Should not show adverse drug interactions with other antimicrobial drugs
• Should have no/short withdrawal time in food-producing animals
• Should have long shelf-life
• It should be easily available and cheap.

Classification:

Classified on the basis of mechanism of action, chemical structure, types of organism affected, antimicrobial spectra, types of action, sources etc.

Mechanism of action

1. Agents inhibiting cell wall synthesis: penicillins, cephalosporins, cycloserine, bacitracin, vancomycin and clotrimazole
2. Agents inhibiting cytoplasmic membrane function: polymyxins, amphotericin B and nystatin.
3. Agents inhibiting protein synthesis: chloramphenicol, tetracyclines, macrolides and aminoglycosides
4. Agents affecting nucleic acid metabolism and synthesis: quinolones, rifampicin, idoxuridine and aciclovir
5. Agents interfering with intermediary metabolism: sulphonamides, trimethoprim and sulphones.

Chemical structure:

1. Sulphonamides: sulphadimidine, sulphadiazine, sulphanilamide and sulphaquinoxaline
2. Diaminopyrimidines: trimethoprim, ormetoprim and baquitoprim
3. Quinolones: nalidixic acid, enrofloxacin, difloxacin and ciprofloxacin
4. β-lactam antibiotics: penicillin G, ampicillin, cloxacilin, cefazolin and cefalexin.
5. Aminoglycosides: streptomycin, gentamicin, amikacin and tobramycin
6. Tetracyclines: oxytetracycline, tetracycline, doxycycline and minocycline
7. Macrolide antibiotics: erythromycin, azithromycin
8. Polypeptide antibiotics: polymixin B, colistin and bacitracin
9. Nitrofuran derivatives: nitrofurantoin, and furazolidone
10. Nitroimidazoles: metronidazole and tinidazole
11. Polyene antibiotics: nystatin and amphotericin B
12. Imidazole derivatives: ketoconazole, fluconazole and clotrimazole

Type of organism/therapeutic use:

1. Antibacterials: penicillins, aminoglycosides, tetracyclines and chloramphenicol
2. Antifungal: amphotericin B, griseofulvin and ketoconazole
3. Antiviral: idoxuridine, vidarabine, zidovudine and ribavirin
4. Antiprotozoal: metronidazole, quinapyramine & diminazene
5. Antihelmintics: albendazole, levamisole, niclosamide & praziquantel
6. Ectoparasiticides: cypermethrin, lindane, amitraz & ethion

Spectrum of activity:

1. Narrow spectrum: penicillin G, streptomycin, erythromycin and vancomycin
2. Broad-spectrum: tetracyclines, chloramphenicol, cefalexin, gentamicin & ampicillin

Type of action:

1. Bacteriostatic: sulphonamides, chloramphenicol, erythromycin, trimethoprim & clindamycin
2. Bactericidal: penicillin G, cefalexin, streptomycin, vancomycin, bacitracin & potentiated sulphonamides.

Source:

a. Natural and semi-synthetic:

1. Fungi: penicillin, griseofulvin, cefalexin
2. Actinomycetes: streptomycin, tetracycline, erythromycin, chloramphenicol
3. Bacteria: polymyxin B, colistin, bacitracin

b. Synthetic: sulphonamides, trimethoprim, quinolones, nitrofurans & nitroimidazoles