Ampicillin

KEY POINTS

• Ampicillin is an amino-penicillin and beta-lactam antibiotic that works by inhibiting the bacterial cell wall in susceptible organisms
• Available as an oral or injectable, but amoxicillin is the more commonly employed oral amino-penicillin
• Has activity versus Gram positives, Gram negatives and anaerobes
  • Does not work against penicillin-resistant Staphylococcus aureus
    • Most MSSA is penicillin-resistant
  • A good drug for beta-hemolytic Streptococci (e.g., Strep. pyogenes & Strep. agalactiae)
  • Beware resistance with alpha-hemolytic Streptococci (e.g., Strep. pneumoniae & Viridans Streptococci)
  • Aminopenicillins are widely considered the drugs of choice for susceptible Enterococci and Listeria monocytogenes
    • Common to combine ampicillin IV with ceftriaxone IV when treating Enterococcal infective endocarditis that is ampicillin-sesntive
  • Many Gram negative anaerobes are resistant to ampicillin alone, add beta-lactamase inhibitor sulbactam to expand this coverage and hit susceptible Bacteroides fragilis
    • Note ampicillin-sulbactam is brand name Unasyn
    • Amp-sulb has activity versus MSSA, but ampicillin alone only has activity against penicillin-sensitive Staphylococcus aureus
• Dose range is typically 1gm every 6 hours to 2gm every 4 hours
  • Requires dosage adjustment for renal insufficiency
  • Short stability of the IV formulation can be problematic, limiting options for extended or continuous infusion strategies
• Beware hypersensitivity reaction and anaphylaxis
  • Penicillin skin testing can assist in ruling out most unclear penicillin allergy labels

RESOURCES

• Injectable Ampicillin Package Insert
• Oral Ampicillin Package Insert
• IDSA Practice Guidelines
• Surgical Prophylaxis Guidelines
• Penicillins Review (Mayo Clinic Proceedings 1999)
• Ampicillin Plus Ceftriaxone For Enterococcal Infective Endocarditis (CID 2013)