What Are Antibiotics?
The word antibiotic originates from the term antibiosis, meaning “against life.” Traditionally, antibiotics were thought to be natural products produced by microorganisms that destroy or inhibit the growth of other microorganisms. Today, the definition has expanded to include synthetic or semi-synthetic antimicrobial compounds. Antibiotics may be bactericidal (kill bacteria) or bacteriostatic (inhibit bacterial growth). Although commonly called “antibacterial drugs,” antibiotics are further categorized to treat bacteria, fungi, or viruses.
Classification of Antibiotics:
Antibiotics are grouped based on their chemical structure and mechanism of action:
- β-Lactam Antibiotics: Penicillin, Cephalosporins, Carbapenems, Monobactams
- Tetracyclines: Doxycycline, Minocycline
- Macrolides: Erythromycin, Azithromycin
- Aminoglycosides: Streptomycin, Gentamicin
- Miscellaneous Agents: Chloramphenicol, Clindamycin, Rifampicin, Fusidic Acid, Griseofulvin
β-Lactam Antibiotics:
β-Lactam antibiotics contain a highly reactive four-membered ring called the β-lactam nucleus. They are the most widely prescribed antibiotics and work by interrupting bacterial cell wall synthesis. They are often combined with β-lactamase inhibitors (e.g., clavulanic acid) to overcome bacterial resistance.
Examples of β-Lactam Antibiotics
Penicillin G (Benzyl Penicillin):
- A natural penicillin with poor oral absorption; therefore given by injection. It binds to penicillin-binding proteins (PBPs), blocks cell wall synthesis, and leads to cell lysis.
- Uses: Meningitis, endocarditis, severe pneumonia, septicaemia.
- Storage: Refrigerate at 2–8°C; do not freeze.
- Formulations: Injection powder, solution.
- Brands: Pfizerpen, Bicillin L-A.
Amoxicillin:
- A broad-spectrum penicillin effective in respiratory, ENT, urinary, and skin infections. It inhibits PBPs and weakens the bacterial cell wall, promoting the bactericidal effect.
- Uses: Bronchitis, tonsillitis, pneumonia, H. pylori ulcers (with other drugs).
- Storage: Tablets at room temperature; liquid discarded after 14 days.
- Formulations: Tablets, suspension.
- Brands: Amoxil, Moxatag.
Cloxacillin:
- Cloxacillin is resistant to β-lactamase produced by staphylococci. It blocks PBPs and promotes autolytic enzyme activity, leading to bacterial cell lysis.
- Uses: Streptococcal and staphylococcal infections.
- Storage: Capsules kept at room temperature; oral liquid refrigerated.
- Formulations: Powder, liquid.
- Brands: Cloxapen, Tegopen.
Aminoglycosides:
Aminoglycosides contain modified amino sugars attached to a central ring. They are effective against many gram-negative bacteria and work by blocking protein synthesis through irreversible binding to the 30S ribosomal subunit.
Example – Streptomycin:
- Streptomycin binds to the 16S rRNA on the 30S ribosomal subunit, causing misreading of mRNA and production of toxic non-functional proteins.
- Uses: Tuberculosis, plague, tularemia, brucellosis.
- Storage: Refrigerate after reconstitution; powder kept at room temperature.
- Formulation: Powder for injection.
- Brands: Agrimycin-17, Brucella.
Tetracyclines:
Tetracyclines are broad-spectrum antibiotics active against gram-positive and gram-negative bacteria. They are widely used in respiratory, urinary, and sexually transmitted infections.
Doxycycline:
- Doxycycline blocks protein synthesis by binding to the 30S ribosomal subunit and preventing tRNA attachment. It is less toxic than first-generation tetracyclines.
- Uses: Rickettsial infections, STDs, respiratory infections, Lyme disease.
- Storage: Store tablets and capsules at 15–30°C.
- Brands: Vibramycin, Doryx, Oracea.
Macrolides:
Macrolides contain a large lactone ring attached to deoxy sugars. They block protein synthesis by binding to the 50S ribosomal subunit and are popular alternatives for patients allergic to penicillin.
Erythromycin:
- Erythromycin binds to 23S rRNA of the 50S ribosome and blocks translocation during protein synthesis.
- Uses: Pneumonia, bronchitis, whooping cough, diphtheria, syphilis.
- Storage: Store below 30°C; protect from moisture.
- Brands: Erythrocin, E-Mycin.
Azithromycin:
- Azithromycin has a long half-life and excellent tissue penetration. It binds to 23S rRNA and blocks protein synthesis.
- Uses: Sinusitis, pneumonia, ear infections, STDs.
- Storage: Keep at room temperature; discard suspension after 10 days.
- Brands: Zithromax, Zmax.
Miscellaneous Antibiotics
Chloramphenicol:
- Chloramphenicol is a broad-spectrum bacteriostatic antibiotic. It inhibits protein synthesis by blocking peptidyl transferase on the 50S subunit. Due to toxicity, it is now reserved for severe infections.
- Uses: Enteric fever, meningitis (in sensitive patients), eye infections.
- Storage: Store eye drops in the refrigerator.
- Brands: Chloromycetin, Ocu-Chlor.
Clindamycin:
- Clindamycin is a semi-synthetic lincosamide that binds to the 50S ribosomal subunit and blocks protein synthesis. It is effective against anaerobes and skin infections including acne.
- Uses: Dental infections, pelvic infections, bone infections, skin acne.
- Storage: Store at room temperature; do not refrigerate liquid form.
- Brands: Cleocin, Cleocin Phosphate.
