Antibiotics are antimicrobial substances that kill or inhibit the growth of bacteria through selective targeting of bacterial structures and metabolic pathways that differ from those of mammalian cells. They are classified by their mechanism of action, spectrum of activity, and chemical structure, with appropriate selection guided by the suspected or confirmed pathogen, site of infection, and patient factors.
What Are Antibiotics?
The discovery and clinical application of antibiotics represent one of the most transformative advances in medical history, converting previously fatal bacterial infections into treatable conditions. However, the emergence and spread of antibiotic resistance threaten this achievement, making antibiotic stewardship and the development of novel agents urgent priorities. Understanding antibiotic mechanisms is essential for rational prescribing, anticipating resistance patterns, and predicting drug interactions.
Mechanism of Action
Cell wall synthesis inhibitors include beta-lactams and glycopeptides. Beta-lactam antibiotics such as penicillins, cephalosporins, carbapenems, and monobactams bind to penicillin-binding proteins, inhibiting transpeptidase enzymes that crosslink peptidoglycan strands during bacterial cell wall synthesis. This weakens the cell wall and leads to osmotic lysis. Beta-lactamases are bacterial enzymes that hydrolyze beta-lactam rings, conferring resistance that is overcome by beta-lactamase inhibitors such as clavulanic acid and tazobactam. Vancomycin, a glycopeptide, binds to the D-alanyl-D-alanine terminus of peptidoglycan precursors, blocking crosslinking through a different mechanism.
Protein synthesis inhibitors target bacterial ribosomes, which are structurally distinct from human ribosomes, providing selective toxicity. Aminoglycosides such as gentamicin bind to the 30S ribosomal subunit, causing mRNA misreading and incorporation of incorrect amino acids. Tetracyclines also bind the 30S subunit, blocking aminoacyl-tRNA binding. Macrolides such as azithromycin bind the 50S subunit and inhibit peptide chain elongation. Linezolid, an oxazolidinone, inhibits formation of the 70S initiation complex.
DNA and RNA synthesis inhibitors include fluoroquinolones such as ciprofloxacin and levofloxacin, which inhibit bacterial DNA gyrase and topoisomerase IV, preventing DNA supercoiling and replication. Rifampin inhibits bacterial DNA-dependent RNA polymerase, blocking transcription. Metronidazole undergoes reduction in anaerobic bacteria, producing toxic compounds that damage bacterial DNA.
Folate antagonists include sulfonamides, which competitively inhibit dihydropteroate synthase, and trimethoprim, which inhibits dihydrofolate reductase. The combination trimethoprim-sulfamethoxazole provides sequential blockade of folate synthesis, synergistically inhibiting bacterial nucleotide synthesis.
Therapeutic Uses
Antibiotics are used for treatment and prophylaxis of bacterial infections. Selection depends on the likely pathogen, local resistance patterns, infection site, and patient characteristics. Empiric therapy is begun before culture results are available, then narrowed based on susceptibility testing. Outpatient respiratory, urinary, and skin infections are common indications. Hospital-acquired infections often require broader-spectrum agents.
Adverse Effects
Beta-lactams cause hypersensitivity reactions ranging from rash to anaphylaxis. Aminoglycosides cause nephrotoxicity and ototoxicity. Fluoroquinolones are associated with tendonitis and tendon rupture, peripheral neuropathy, and QT prolongation. Clostridium difficile colitis results from disruption of the intestinal microbiome and is associated with many antibiotics, particularly clindamycin, cephalosporins, and fluoroquinolones.
Contraindications
Penicillin allergy is common and requires avoidance of beta-lactams or cautious challenge. Fluoroquinolones are relatively contraindicated in children and pregnant women due to cartilage toxicity in animal studies. Tetracyclines are contraindicated in children under eight years and in pregnancy due to effects on bone and tooth development.
Conclusion
Antibiotics remain essential for treating bacterial infections, but their effectiveness is increasingly compromised by resistance. Appropriate prescribing, including correct dose, duration, and spectrum of activity, is critical for preserving these valuable therapeutic resources for future patients.
