How are antibiotics classified?

Antibiotic Classification

Antibiotics are primarily classified according to the WHO AWaRe framework into Access, Watch, and Reserve categories based on their potential for resistance development and priority for use, with additional classifications based on their mechanism of action and chemical structure. 1

WHO AWaRe Classification

The World Health Organization (WHO) classifies antibiotics into three main categories:

1. Access (Green) - First-line antibiotics for common infections with lower resistance potential

   • Examples: Penicillins, amoxicillin, amoxicillin-clavulanate 1

2. Watch (Orange) - Antibiotics with higher resistance risk

   • Examples: Fluoroquinolones, carbapenems 1

3. Reserve (Red) - Last-resort antibiotics for multidrug-resistant infections 1

Classification by Mechanism of Action

Antibiotics can be categorized based on how they kill or inhibit bacterial growth:

1. Cell Wall Synthesis Inhibitors

   • β-lactams (penicillins, cephalosporins, carbapenems, monobactams) 2
   • Glycopeptides (vancomycin)
   • Fosfomycin 3

2. Protein Synthesis Inhibitors

   • Tetracyclines
   • Macrolides
   • Aminoglycosides
   • Lincosamides (clindamycin) 3

3. Nucleic Acid Synthesis Inhibitors

   • DNA synthesis inhibitors (fluoroquinolones) 3
   • RNA synthesis inhibitors (rifampin) 3

4. Membrane-Active Antibiotics

   • Polymyxins (colistin) 4

β-Lactam Antibiotics Classification

β-Lactams are further classified into:

1. Penicillins

   • Natural penicillins (Penicillin G)
   • Aminopenicillins (Amoxicillin)
   • Anti-staphylococcal penicillins (Oxacillin, Flucloxacillin)
   • Extended-spectrum penicillins with β-lactamase inhibitors (Amoxicillin-clavulanate) 2

2. Cephalosporins (by generation)

   • First-generation: Cefazolin, Cefalexin
   • Second-generation: Cefuroxime, Cefoxitin
   • Third-generation: Ceftriaxone, Cefotaxime, Ceftazidime
   • Fourth-generation: Cefepime 1, 5

3. Carbapenems

   • Imipenem, Meropenem, Ertapenem 1

4. Monobactams

   • Aztreonam 2

Clinical Application Based on Pathogens

When selecting antibiotics, consider the pathogen and its susceptibility pattern:

• Streptococcus pneumoniae: Penicillin G, Amoxicillin, Amoxicillin-clavulanate 1
• Methicillin-susceptible Staphylococcus aureus: Oxacillin, Cefazolin, Flucloxacillin 1
• Methicillin-resistant Staphylococcus aureus: Vancomycin with rifampin, Linezolid 1
• Pseudomonas aeruginosa: Anti-pseudomonal beta-lactams (Ceftazidime, Cefoperazone) 1

Important Considerations in Antibiotic Selection

• Spectrum of activity: Match the antibiotic's spectrum to the suspected pathogen 1
• Local resistance patterns: Consider regional antibiotic resistance data
• Site of infection: Some antibiotics penetrate certain tissues better than others
• Patient factors: Consider allergies, renal/hepatic function, and comorbidities 1

Antimicrobial Resistance Considerations

• Broad-spectrum antibiotics contribute more significantly to resistance development
• Optimizing dosage and duration helps minimize resistance emergence 1
• Understanding cross-resistance within antibiotic classes is crucial for effective therapy 6

Clinical Susceptibility Categories

Bacteria are categorized as:

• Susceptible: Likely to respond to standard antibiotic dosing
• Intermediate: May respond to higher doses or if the antibiotic concentrates at the infection site
• Resistant: Unlikely to respond even to maximum doses 6

Understanding these classifications helps clinicians select appropriate antibiotics for specific infections while considering the risk of antimicrobial resistance.