Ceftriaxone Injection Spectrum of Activity
Ceftriaxone is a broad-spectrum, third-generation cephalosporin with potent activity against most Gram-negative bacteria and moderate activity against Gram-positive organisms, but it is notably less active than first- and second-generation cephalosporins against many Gram-positive pathogens. 1, 2
Gram-Negative Coverage
Ceftriaxone demonstrates excellent activity against common Gram-negative respiratory and urinary pathogens:
- Highly effective against Haemophilus influenzae (including β-lactamase producing strains), with 100% susceptibility maintained through 2000 3
- Potent activity against Neisseria gonorrhoeae, with 100% susceptibility including penicillinase-producing strains, achieving 98.9% cure rates for uncomplicated urogenital and anorectal infections 4, 3
- Strong activity against Enterobacteriaceae including Escherichia coli (0.2-0.4% resistance), Klebsiella pneumoniae (1.9-2.6% resistance), and Proteus mirabilis (0.2-0.3% resistance) 3
- Effective against Moraxella catarrhalis with 99.7% susceptibility 3
Important limitations in Gram-negative coverage:
- Limited activity against Pseudomonas aeruginosa - cannot be recommended as sole therapy for pseudomonal infections 2, 5
- Moderate resistance among Enterobacter cloacae (21.7-23.9% resistance) 3
- Increasing resistance in Acinetobacter species (24.8% in 1996 rising to 45.1% by 2000) 3
Gram-Positive Coverage
Ceftriaxone has moderate activity against Gram-positive organisms, but is generally less potent than earlier-generation cephalosporins:
- Moderate activity against Streptococcus pneumoniae, with 5.0-6.6% resistance rates, though this is comparable to second-generation agents like cefuroxime 4, 3
- Limited activity against drug-resistant S. pneumoniae (DRSP) - similar to other third-generation oral cephalosporins 4
- Good activity against methicillin-susceptible Staphylococcus aureus (MSSA) with only 0.1-0.3% resistance, though cefazolin is preferred by IDSA guidelines for MSSA infections 6, 3
- Excellent activity against Streptococcus pyogenes and group B streptococci with 0% resistance 3
- No activity against methicillin-resistant Staphylococcus aureus (MRSA) 4
Anaerobic Coverage
Ceftriaxone has some activity against anaerobic bacteria:
- Limited activity against Bacteroides fragilis and other anaerobes, requiring combination with metronidazole for intra-abdominal infections 6, 7
Clinical Context and Comparative Spectrum
Ceftriaxone's structural analog cefpodoxime proxetil demonstrates similar activity against respiratory pathogens 4
The broader spectrum of ceftriaxone compared to narrower agents like cefazolin comes with increased risk of antimicrobial resistance and collateral damage to normal flora 6
Common Pitfalls
- Do not use ceftriaxone as monotherapy for Pseudomonas aeruginosa infections - it lacks sufficient activity 2, 5
- Avoid using ceftriaxone when narrower-spectrum agents would suffice (e.g., cefazolin for MSSA) to minimize resistance development 6
- Do not rely on ceftriaxone alone for intra-abdominal infections - combination with metronidazole is necessary for anaerobic coverage 6
- Be aware of increasing resistance in Acinetobacter species when considering empiric therapy 3