Chapter 35  ·  Module 3  ·  Antibacterial Agents
Carbapenems, Monobactams & CRE
Spectrum, key differences, aztreonam, and resistance therapy at a glance
Abbreviations: MSSA = methicillin-susceptible Staphylococcus aureus  ·  MRSA = methicillin-resistant Staphylococcus aureus  ·  CRE = carbapenem-resistant Enterobacteriaceae  ·  KPC = Klebsiella pneumoniae carbapenemase  ·  NDM = New Delhi metallo-beta-lactamase  ·  ESBL = extended-spectrum beta-lactamase  ·  DHP-I = dehydropeptidase I  ·  PBP = penicillin-binding protein
Carbapenem Comparison
Agent Pseudomonas Formulation Note Seizure Risk Key Niche
Imipenem-cilastatin Yes (less than meropenem) Requires cilastatin (DHP-I inhibitor) Highest (1–3%); avoid in central nervous system disease Broad gram-positive + gram-negative; avoid in meningitis
Meropenem Yes (reference) No cilastatin needed Low Gram-negative meningitis; ESBL bacteremia; extended infusion for Pseudomonas
Ertapenem No Once-daily (long half-life) Low ESBL without Pseudomonas; outpatient parenteral therapy; step-down
Doripenem Yes Standard infusion Low Limited current use; complicated urinary tract infections, complicated intra-abdominal infections
Aztreonam — The Monobactam
Aztreonam: Monocyclic Beta-Lactam — Gram-Negative Only
Spectrum
Gram-negative only (Enterobacteriaceae, Pseudomonas, H. influenzae). No gram-positive or anaerobic activity.
Penicillin Allergy
Safe in penicillin-allergic patients. Exception: cross-reacts with ceftazidime (shared side chain).
NDM Role
Not hydrolyzed by metallo-beta-lactamases. Aztreonam-avibactam: key option for NDM-producing organisms.
CRE Therapy by Resistance Mechanism
Class A Carbapenemase
KPC Producers
  • Ceftazidime-avibactam
  • Meropenem-vaborbactam
  • Imipenem-relebactam
  • Cefiderocol (salvage)
Class B Metallo-Beta-Lactamase
NDM / VIM / IMP Producers
  • Aztreonam-avibactam
  • Aztreonam + ceftazidime-avibactam
  • Cefiderocol (salvage)
  • Not covered: all carbapenem combinations above
Class D Serine Carbapenemase
OXA-48 Producers
  • Ceftazidime-avibactam (covers OXA-48)
  • Confirm OXA-48 vs. metallo-enzyme
  • Meropenem-vaborbactam: does not cover OXA-48
OXA-23/58 Carbapenemase
Carbapenem-Resistant Acinetobacter baumannii
  • Sulbactam-durlobactam (first approved agent)
  • Cefiderocol (alternative)
  • Polymyxins + combination regimens
Key Rule: Genotype Before Therapy

No single agent covers all carbapenem-resistant organisms. Distinguish KPC from NDM from OXA-48 by phenotypic or genotypic testing before committing to definitive therapy. Ceftazidime-avibactam covers KPC and OXA-48 but fails against NDM. Aztreonam-avibactam is required for NDM.