What is the coverage and dosage of ampicillin (Ampicillin) for treating infections?

Ampicillin Coverage and Dosing Guidelines

Ampicillin is a broad-spectrum aminopenicillin effective against many Gram-positive, some Gram-negative, and anaerobic bacteria, with dosing regimens that vary by infection type, severity, and patient population.

Antimicrobial Coverage

• Ampicillin is effective against many streptococci, including Group A, B, C, G streptococci and some viridans streptococci 1, 2
• It covers enterococci when combined with gentamicin for synergistic effect 1
• Some coverage against Haemophilus influenzae, though increasing resistance has been noted 2, 3
• Effective against many strains of Escherichia coli, Proteus mirabilis, and other susceptible Gram-negative organisms 4, 5
• Active against Leptospira species 6
• Limited coverage against Staphylococcus aureus (only penicillin-susceptible strains) 1
• Not effective against MRSA, Pseudomonas species, or beta-lactamase producing organisms unless combined with a beta-lactamase inhibitor (e.g., sulbactam) 7

Adult Dosing Guidelines

Oral Administration

• For genitourinary or gastrointestinal tract infections: 500 mg four times daily in equally spaced doses 8
• For respiratory tract infections: 250 mg four times daily in equally spaced doses 8
• For gonorrhea: Single oral dose of 3.5 grams administered simultaneously with 1 gram of probenecid 8
• Should be administered at least 30 minutes before or 2 hours after meals for maximal absorption 8

Intravenous Administration

• For infective endocarditis: 200-300 mg/kg/day IV divided every 4-6 hours (up to 12 g daily) 1, 2
• For HACEK group infections: 200-300 mg/kg/day IV divided every 4-6 hours (up to 12 g daily), though ceftriaxone is preferred due to increasing β-lactamase production 2
• For uncomplicated urinary tract infections: 18-30 g/day IV in divided doses 2

Pediatric Dosing Guidelines

• For children weighing 20 kg or less with genitourinary or GI tract infections: 100 mg/kg/day total, divided into four equal doses 8
• For children weighing 20 kg or less with respiratory tract infections: 50 mg/kg/day total, divided into 3-4 equal doses 8
• For infective endocarditis in children: 200-300 mg/kg/day IV divided every 4-6 hours (up to 12 g daily) 1
• For neonates and infants (gestational age >34 weeks):
  • Postnatal age ≤7 days: 50 mg/kg/dose IV every 12 hours 2
  • Postnatal age >7 days: 75 mg/kg/dose IV every 12 hours 2

Duration of Therapy

• For uncomplicated urinary tract infections: 3-7 days 2, 9
• For streptococcal pharyngitis: 10 days to prevent rheumatic fever or glomerulonephritis 9, 8
• For infective endocarditis: 4-6 weeks, often 6-8 weeks 1, 2
• For general infections: Continue for at least 48-72 hours after patient becomes asymptomatic or evidence of bacterial eradication is obtained 8

Special Considerations and Caveats

• When treating enterococcal infections, combination with gentamicin is recommended for the first 2 weeks or for the entire course for enterococcal endocarditis 1, 2
• For bacterial meningitis, particularly H. influenzae, high-dose regimens (400 mg/kg/day) have not shown significant benefit over lower doses (150 mg/kg/day) 3
• Ampicillin/sulbactam combination provides broader coverage, particularly for beta-lactamase producing organisms and some Acinetobacter baumannii strains 7
• Resistance patterns should be considered when selecting ampicillin, especially for Gram-negative infections 9
• For chronic infections, frequent bacteriologic and clinical assessment is necessary during therapy and potentially for several months afterward 8

Monitoring

• For endocarditis treatment, blood cultures should be obtained after completion of antibiotic treatment, though this may sometimes result in isolation of contaminants 1
• For gonorrhea treatment, follow-up cultures should be obtained from original site(s) of infection 7-14 days after therapy; in women, cultures from both endocervical and anal canals are recommended 8