What is the antimicrobial spectrum of ceftriaxone?

Ceftriaxone Antimicrobial Coverage

Spectrum of Activity

Ceftriaxone is a broad-spectrum third-generation cephalosporin with excellent activity against most Gram-positive cocci (except MRSA and enterococci), comprehensive coverage of Gram-negative organisms (including Enterobacteriaceae), and notable activity against Neisseria and Haemophilus species, while lacking reliable activity against Pseudomonas aeruginosa and anaerobes like Bacteroides fragilis. 1

Gram-Positive Coverage

Excellent Activity

• Streptococcus pneumoniae: Highly active with MIC90 ≤0.07 mcg/mL for susceptible strains 2. Cures >99% of penicillin-susceptible and intermediate strains, though activity decreases to 80% against penicillin-resistant isolates 3. Resistance rates remained stable at 5.0-6.6% from 1996-2000 4.
• Beta-hemolytic streptococci (Groups A, B, C, G): Exquisitely susceptible with >99% susceptibility rates 3. No beta-lactam-resistant S. pyogenes or Group B streptococci identified 4.
• Viridans group streptococci: Good activity with MIC90 ≤0.07 mcg/mL, though routinely less susceptible than other streptococci 3, 2.

Good Activity

• Methicillin-susceptible Staphylococcus aureus (MSSA): Active with MIC90 ≤5 mcg/mL and resistance rates of only 0.1-0.3% 4, 2. Cures >98% of infections caused by oxacillin-susceptible staphylococci 3.
• Coagulase-negative staphylococci: Similar activity to MSSA when methicillin-susceptible 3.

No Activity

• Methicillin-resistant staphylococci (MRSA/MRSE): Generally inactive 2.
• Enterococcus species: No activity 1, 2.

Gram-Negative Coverage

Excellent Activity

• Neisseria gonorrhoeae: 100% susceptibility with no ceftriaxone-resistant strains reported 5, 4. Single 125 mg IM dose cures 99.1% of uncomplicated urogenital/anorectal infections and ≥90% of pharyngeal infections 5.
• Neisseria meningitidis: Highly active with MIC90 ≤0.024 mcg/mL 2. Recommended for meningococcal meningitis and sepsis 5.
• Haemophilus influenzae: 100% susceptibility including beta-lactamase producing strains, with MIC90 ≤0.024 mcg/mL 4, 2. Effective for meningitis, otitis media, and respiratory infections 1.
• Moraxella catarrhalis: 99.7% susceptibility including beta-lactamase producing strains 4.

Good Activity Against Enterobacteriaceae

• Escherichia coli: Resistance rates 0.2-0.4% with consistent activity 4.
• Proteus mirabilis: Resistance rates 0.2-0.3% 4.
• Klebsiella pneumoniae: Resistance rates 1.9-2.6% 4.
• Klebsiella oxytoca: Resistance rates 3.5-4.8% 4.
• Morganella morganii: Resistance rates 0.3-2.1% 4.
• Serratia marcescens: Resistance rates 1.6-3.8% 4.

Moderate/Variable Activity

• Enterobacter cloacae: Higher resistance rates of 21.7-23.9% due to chromosomal beta-lactamase production 4. Caution advised due to risk of derepressed mutants 6.
• Pseudomonas aeruginosa: Moderate activity with MIC50 12-28 mcg/mL 2. Not reliably covered; alternative agents preferred 1, 7.
• Acinetobacter species: Increasing resistance from 24.8% (1996) to 45.1% (2000) 4.

Important Resistance Considerations

• Extended-spectrum beta-lactamases (ESBLs): Diminished activity against ESBL-producing Enterobacteriaceae; meropenem recommended when suspected 5, 6.
• AmpC hyperproducers: Reduced effectiveness against derepressed mutants of Enterobacter, Citrobacter, and Serratia species 6.

Anaerobic Coverage

Variable Activity

• Peptostreptococcus species: Active 1.
• Clostridium species: Variable activity; most C. difficile strains are resistant 1.
• Bacteroides fragilis: Generally inactive with MIC >64 mcg/mL 2. Poor coverage of Gram-negative anaerobes 5.

Clinical Applications by Indication

FDA-Approved Indications 1

• Lower respiratory tract infections: Active against S. pneumoniae, S. aureus, H. influenzae, H. parainfluenzae, K. pneumoniae, E. coli, E. aerogenes, P. mirabilis, S. marcescens.
• Acute bacterial otitis media: Covers S. pneumoniae, H. influenzae (including beta-lactamase producers), M. catarrhalis (including beta-lactamase producers).
• Uncomplicated gonorrhea: Cervical, urethral, rectal, and pharyngeal infections caused by N. gonorrhoeae (penicillinase and non-penicillinase producing).
• Meningitis: H. influenzae, N. meningitidis, S. pneumoniae 1. Also effective against S. epidermidis and E. coli meningitis in limited cases 1.
• Bacterial septicemia: S. aureus, S. pneumoniae, E. coli, H. influenzae, K. pneumoniae.
• Skin/soft tissue infections: Broad coverage including staphylococci, streptococci, Enterobacteriaceae, and some anaerobes.
• Urinary tract infections: E. coli, P. mirabilis, P. vulgaris, M. morganii, K. pneumoniae.
• Pelvic inflammatory disease: N. gonorrhoeae (requires addition of anti-chlamydial coverage) 1.
• Intra-abdominal infections: E. coli, K. pneumoniae, B. fragilis, Clostridium species, Peptostreptococcus species.
• Bone/joint infections: S. aureus, S. pneumoniae, E. coli, P. mirabilis, K. pneumoniae, Enterobacter species.

Key Coverage Gaps

• No activity against: MRSA, enterococci, Chlamydia trachomatis 1.
• Limited activity against: P. aeruginosa, B. fragilis, ESBL-producing organisms 6, 2.
• Requires combination therapy for: Polymicrobial infections involving anaerobes, suspected ESBL producers, or when atypical coverage needed 5, 1.