What are the most common antibiotics, their classes, mechanisms of action, side effects, and uses?

Common Antibiotics: Classes, Mechanisms, Side Effects, and Uses

Beta-Lactam Antibiotics

Penicillins

• Mechanism: Inhibit bacterial cell wall synthesis by disrupting peptidoglycan formation, specifically interfering with transpeptidase and DD-carboxypeptidase enzymes that link peptide chains in the murein structure 1
• Common agents: Penicillin G, amoxicillin, amoxicillin-clavulanate, ampicillin 2
• Primary uses: First-line for streptococcal infections, skin infections, respiratory tract infections 2; amoxicillin recommended for community-acquired pneumonia in healthy adults 3
• Side effects: Hypersensitivity reactions ranging from mild drug eruptions to anaphylaxis; gastrointestinal disturbances including nausea, diarrhea, and abdominal discomfort 4

Cephalosporins

• Mechanism: Cell wall synthesis inhibition similar to penicillins 5
• Classification by generation 2:
  • First-generation (cefazolin, cefalexin): Good activity against staphylococci and streptococci 3
  • Second-generation (cefuroxime): Expanded gram-negative coverage 2
  • Third-generation (ceftriaxone, cefotaxime): Broader gram-negative spectrum 2
  • Fourth-generation (cefepime): Enhanced gram-positive and gram-negative coverage 2
• Primary uses: Vary by generation; first-generation for skin infections, third-generation for severe respiratory and systemic infections 2
• Side effects: Hypersensitivity reactions, gastrointestinal disturbances including nausea, diarrhea, and abdominal discomfort 4

Carbapenems

• Common agents: Meropenem, imipenem 2
• Mechanism: Cell wall synthesis inhibition with broadest spectrum of beta-lactam activity 5
• Primary uses: Severe or multi-drug resistant infections, intra-abdominal infections, necrotizing fasciitis (in combination therapy) 2
• Side effects: Similar to other beta-lactams with hypersensitivity reactions and gastrointestinal effects 4

Protein Synthesis Inhibitors

Tetracyclines

• Common agents: Doxycycline, minocycline 2
• Mechanism: Inhibit bacterial protein synthesis by binding to 30S ribosomal subunit 5
• Primary uses: Atypical infections, rickettsial diseases, skin infections 2; first-line option for community-acquired pneumonia 3; recommended for COPD exacerbations 3
• Side effects: Doxycycline causes photosensitivity and gastrointestinal disturbances (dose-dependent); minocycline associated with tinnitus, dizziness, and pigment deposition of skin, mucous membranes, and teeth 4; tooth discoloration in children 4
• Important caveat: Minocycline has higher rate of serious adverse events (8.8 cases per 100,000 patient-years) including DRESS, drug-induced lupus, and pseudotumor cerebri 4

Macrolides

• Common agents: Azithromycin, clarithromycin, erythromycin 2
• Mechanism: Inhibit protein synthesis by binding to bacterial ribosomes 5
• Primary uses: Respiratory infections, particularly community-acquired pneumonia; effective against gram-positive bacteria and atypical pathogens 2; recommended for CAP in healthy adults and as combination therapy with beta-lactams in patients with comorbidities 3
• Side effects: Gastrointestinal disturbances (erythromycin > azithromycin); cardiac conduction abnormalities; rarely hepatotoxicity; azithromycin associated with cutaneous hypersensitivity reactions 4

Aminoglycosides

• Common agents: Gentamicin, tobramycin, amikacin 2
• Mechanism: Bind to 30S ribosomal subunit causing misreading of mRNA and inhibition of protein synthesis; specifically, the 3''OH function reacts with lysine from S12 protein 1
• Primary uses: Serious gram-negative infections including Pseudomonas aeruginosa, often used in combination with beta-lactams 2
• Side effects: Nephrotoxicity and ototoxicity are major concerns 6

Oxazolidinones

• Common agent: Linezolid 2
• Mechanism: Unique mechanism inhibiting protein synthesis 2
• Primary uses: MRSA and VRE infections; skin and soft tissue infections 2
• Side effects: Hematologic toxicity with prolonged use 4

Glycopeptides

• Common agents: Vancomycin, teicoplanin 2
• Mechanism: Inhibit cell wall synthesis by different mechanism than beta-lactams 5
• Primary uses: Serious infections with resistant gram-positive bacteria, including MRSA 2; necrotizing fasciitis in combination therapy 2
• Side effects: Nephrotoxicity, ototoxicity, infusion-related reactions 6

Fluoroquinolones

• Common agents: Ciprofloxacin, levofloxacin, moxifloxacin 2
• Mechanism: Inhibit bacterial DNA synthesis by targeting nucleic acid replication 5
• Primary uses: Respiratory, urinary, and gastrointestinal infections due to broad-spectrum activity 2; effective against most gram-negative pathogens 3; respiratory fluoroquinolones recommended for CAP in patients with comorbidities 3
• Side effects: Tendon rupture, QT prolongation, peripheral neuropathy; potential effects on weight-bearing joints in children 3
• Important caveat: Classified as Watch antibiotics with higher resistance potential and should be targets of stewardship programs 4

Sulfonamides

• Common agent: Trimethoprim-sulfamethoxazole (TMP-SMX) 4
• Mechanism: Inhibit bacterial folate synthesis 5
• Primary uses: Skin and soft tissue infections including purulent infections 3, 2
• Side effects: Gastrointestinal upset, photosensitivity, drug eruptions; severe reactions include Stevens-Johnson syndrome and toxic epidermal necrolysis (higher risk in HIV patients) 4; hematopoietic disorders including neutropenia, agranulocytosis, aplastic anemia, thrombocytopenia 4; fulminant hepatic necrosis 4
• Critical warning: Severe toxicity when combined with methotrexate; patients on long-term therapy require periodic complete blood count monitoring 4

Nitroimidazoles

• Common agent: Metronidazole 4
• Mechanism: Disrupts DNA and inhibits nucleic acid synthesis 5
• Primary uses: Anaerobic infections, intra-abdominal infections (in combination) 2; commonly used in antibiotic cocktails for gut depletion 4
• Side effects: Gastrointestinal disturbances, metallic taste, peripheral neuropathy with prolonged use 6

Glycylcyclines

• Common agent: Tigecycline 7
• Mechanism: Protein synthesis inhibitor, tetracycline derivative 7
• Primary uses: Complicated skin and skin structure infections, complicated intra-abdominal infections, community-acquired bacterial pneumonia 7
• Critical warning: Black box warning for increased all-cause mortality; mortality imbalance and lower cure rates in hospital-acquired pneumonia 7; meta-analysis shows increased mortality association compared to other bacteriostatic agents 8
• Side effects: Hepatic adverse effects, pancreatitis, anaphylactic reactions, tooth discoloration, C. difficile-associated diarrhea 7

Key Clinical Considerations

Antibiotic Stewardship

• Duration: Limit antibiotic use to shortest possible duration, ideally 3-4 months for acne, with concomitant retinoid use 4; 5-day courses recommended for pneumonia and COPD exacerbations 3
• Resistance monitoring: Consider local resistance patterns when selecting empiric therapy 3; Watch antibiotics (fluoroquinolones, carbapenems) should be key targets of stewardship programs 4
• Reserve antibiotics: Eight antibiotics designated as last-resort options for multidrug-resistant bacteria, requiring protection through monitoring programs 4

Common Pitfalls

• C. difficile risk: Monitor for diarrhea with all antibiotics, which may indicate C. difficile infection 3
• Bacteriostatic vs bactericidal: This distinction is clinically irrelevant for pneumonia, skin/soft tissue infections, and abdominal infections 8; however, specific agents like linezolid show better clinical cure rates while tigecycline shows increased mortality 8
• Renal dosing: Doxycycline is primarily hepatically metabolized and safe in renal impairment, unlike other tetracyclines 4