What types of bacterial infections is penicillin (Penicillin) effective against?

Bacteria Covered by Penicillin

Penicillin is most effective against gram-positive organisms, particularly streptococci, pneumococci, and some anaerobes, though increasing resistance patterns limit its use as monotherapy for many infections. 1, 2

Penicillin-Susceptible Organisms

Highly Susceptible Gram-Positive Bacteria

• Streptococci:
  • Group A (S. pyogenes)
  • Group B (S. agalactiae)
  • Groups C, G, H, L, and M streptococci
  • Viridans group streptococci
• Streptococcus pneumoniae (penicillin-susceptible strains)
• Listeria monocytogenes
• Corynebacterium diphtheriae
• Bacillus anthracis (anthrax)
• Clostridia species
• Actinomyces species

Susceptible Gram-Negative Bacteria

• Neisseria meningitidis
• Neisseria gonorrhoeae (non-penicillinase producing strains)

Spirochetes

• Treponema pallidum (syphilis) - extremely susceptible 1
• Leptospira species
• Spirillum minus

Other Organisms

• Streptobacillus moniliformis

Important Considerations Regarding Resistance

Penicillin Resistance

1. Penicillinase-producing bacteria are resistant to penicillin G and penicillin V:

   • Many staphylococcal strains
   • Increasing numbers of Neisseria gonorrhoeae
   • Some Haemophilus influenzae

2. Penicillin-resistant Streptococcus pneumoniae (PRSP):

   • Resistance rates vary geographically
   • For non-meningeal infections with intermediate resistance (MIC 0.12-1 mg/L), high-dose penicillin may still be effective
   • For meningeal infections, ceftriaxone or cefotaxime is preferred 3

3. New susceptibility breakpoints for S. pneumoniae (non-meningitis):

   • Susceptible: MIC ≤2 mg/L
   • Intermediate: MIC 4 mg/L
   • Resistant: MIC ≥8 mg/L 3

Clinical Implications

• For pneumococcal pneumonia, penicillin G at high doses (2g IV q4h) remains effective against strains with MICs ≤8 mg/L 3
• For meningitis, penicillin is not recommended for empiric therapy due to potential resistance
• For skin and soft tissue infections caused by Group A streptococci, penicillin remains the drug of choice 3

Bacteria NOT Covered by Penicillin

• Methicillin-resistant Staphylococcus aureus (MRSA)
• Penicillinase-producing staphylococci
• Most Gram-negative bacilli (including Escherichia coli, Klebsiella, Pseudomonas)
• Enterobacteriaceae
• Anaerobes such as Bacteroides fragilis

Appropriate Dosing for Maximum Effectiveness

For serious infections with susceptible organisms:

• IV Penicillin G: 2-4 million units every 4-6 hours (up to 20-24 million units/day for severe infections) 3, 4
• Oral Penicillin V: Limited to mild-moderate infections due to lower bioavailability 2

Common Pitfalls in Penicillin Use

1. Underestimating resistance: Always consider local resistance patterns when selecting penicillin
2. Inadequate dosing: For serious infections, high-dose therapy is often required
3. Failure to recognize beta-lactamase production: Consider combination with beta-lactamase inhibitors or alternative agents for suspected resistant organisms
4. Overlooking allergic reactions: True penicillin allergies occur in approximately 10% of patients reporting an allergy

In summary, while penicillin remains an important antibiotic with excellent activity against many gram-positive organisms, its use must be guided by local resistance patterns and appropriate dosing to ensure optimal outcomes for patients with bacterial infections.