PENICILLIN CLASSIFICATION

Classification of Penicillin

by

Penicillin are among the earliest discovered and most widely used antibiotics. They belong to the beta-lactam class and are highly effective against various gram-positive and some gram-negative bacteria. Penicillin work by inhibiting bacterial cell wall synthesis and are often used in both clinical and community settings.

This blog from Pharmacy Freak provides a detailed classification, clinical applications, drug of choice highlights, side effects, and practical insights for pharmacy and medical students.

What is Penicillin

Penicillins are beta-lactam antibiotics derived from Penicillium fungi. They act by inhibiting bacterial cell wall synthesis by binding to penicillin-binding proteins (PBPs), leading to bacterial cell lysis.

Classification of Penicillins (KD Tripathi)

  • Natural penicillin: Benzyl penicillin (Penicillin G)
  • Semisynthetic penicillins
    • Acid-resistant alternative to penicillin G: Phenoxymethyl penicillin (Penicillin V)
    • Penicillinase resistant penicillins: Methicillin, Cloxacillin, Dicloxacillin
    • Extended spectrum penicillins –
      • Aminopenicillins: Ampicillin, Bacampicillin, Amoxicillin
      • Ureidopenicillins: Piperacillin, Mezlocillin
      • Carboxypenicillin: Carbenicillin
  • β-lactamase inhibitors: Clavulanic acid, Sulbactam, Tazobactam

Classification of Penicillins (General)

Penicillins are classified based on their spectrum of activity and resistance to beta-lactamases.

  1. Natural Penicillins
    Active primarily against gram-positive cocci and some gram-negative cocci
    Examples:
  • Penicillin G (benzylpenicillin) – IV/IM
  • Penicillin V (phenoxymethylpenicillin) – Oral
    Use: Streptococcal infections, syphilis, meningococcal infections
  1. Penicillinase-Resistant Penicillins (Anti-staphylococcal)
    Resistant to staphylococcal beta-lactamase
    Examples:
  • Methicillin (no longer used due to nephrotoxicity)
  • Nafcillin, Oxacillin, Cloxacillin, Dicloxacillin
    Use: Staphylococcus aureus infections (MSSA)
  1. Aminopenicillins (Extended-spectrum)
    Active against gram-positive bacteria and some gram-negative organisms
    Examples:
  • Ampicillin (IV, oral)
  • Amoxicillin (oral)
    Use: Respiratory tract infections, urinary tract infections, enteric infections
  1. Antipseudomonal Penicillins
    Broader gram-negative activity including Pseudomonas aeruginosa
    Examples:
  • Piperacillin
  • Ticarcillin
    Use: Hospital-acquired infections, pseudomonal infections
  1. Beta-lactamase Inhibitor Combinations
    Combination of penicillins with beta-lactamase inhibitors to extend spectrum
    Examples:
  • Ticarcillin + Clavulanic acid
    Use: Mixed infections, resistant bacterial strains, polymicrobial infections
  • Amoxicillin + Clavulanic acid
  • Ampicillin + Sulbactam
  • Piperacillin + Tazobactam

Uses

Penicillins are used in the treatment of:

  • Streptococcal infections (pharyngitis, cellulitis)
  • Pneumococcal pneumonia
  • Syphilis (Penicillin G)
  • Gonorrhea (in penicillin-sensitive strains)
  • Dental infections
  • Otitis media, sinusitis
  • Urinary tract infections
  • Endocarditis prophylaxis
  • Meningitis (Penicillin G or Ampicillin)
  • Staphylococcal infections (Nafcillin, Oxacillin)
  • Pseudomonal infections (Piperacillin + Tazobactam)

Drug of Choice Highlights

  • Streptococcal pharyngitis – Penicillin V
  • Syphilis – Benzathine Penicillin G
  • Enterococcal infections – Ampicillin
  • Listeria monocytogenes meningitis – Ampicillin
  • Staphylococcus aureus (MSSA) – Cloxacillin or Nafcillin
  • Pseudomonas – Piperacillin + Tazobactam
  • Dog or cat bite infections – Amoxicillin + Clavulanic acid
  • H. pylori (part of triple therapy) – Amoxicillin
  • Dental abscess – Amoxicillin

Side Effects

  • Hypersensitivity reactions (rash, urticaria, anaphylaxis) – most common
  • Gastrointestinal upset (diarrhea, nausea)
  • Pseudomembranous colitis – especially with ampicillin
  • Nephrotoxicity – with methicillin (no longer used)
  • Electrolyte disturbances – with large doses of IV penicillins
  • Superinfections – such as candidiasis

Updated Clinical Pearls

  • Penicillins are time-dependent antibiotics; dosing should aim to maintain plasma levels above the minimum inhibitory concentration (MIC).
  • Combination with beta-lactamase inhibitors extends their spectrum to include many beta-lactamase-producing organisms.
  • Amoxicillin has better oral bioavailability and causes less diarrhea than ampicillin.
  • Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to all beta-lactam antibiotics; alternative agents like vancomycin are used.
  • Penicillin allergy should be confirmed before avoidance; many reported allergies are not true IgE-mediated hypersensitivity.
  • Penicillin G is still the gold standard treatment for syphilis and neurosyphilis.

Trivia

  • Accidental Discovery: Penicillin was discovered accidentally by Sir Alexander Fleming in 1928 when mold (Penicillium notatum) contaminated a petri dish in his lab, leading to one of medicine’s most important breakthroughs.
  • First Successful Antibiotic: Penicillin became the first widely used antibiotic, revolutionizing medical treatment and dramatically reducing deaths from bacterial infections during World War II.
  • Common Allergy: Penicillins are among the most common causes of drug allergies, affecting up to 10% of people. However, most individuals who believe they’re allergic actually lose their sensitivity over time or were misdiagnosed.

References

  1. Tripathi KD. Essentials of Medical Pharmacology. 7th ed. New Delhi: Jaypee Brothers Medical Publishers; 2013. p. 720–730
  2. Gupta S, Garg A. Review of Pharmacology. 15th ed. New Delhi: Jaypee Brothers Medical Publishers; 2023. p. 318–321
  3. Brunton LL, Chabner BA, Knollmann BC, editors. Goodman and Gilman’s The Pharmacological Basis of Therapeutics. 12th ed. New York: McGraw-Hill Education; 2011. p. 1455–1468

Related Links

Leave a Comment