In an adult with normal renal function, what genotype(s) are responsible for carbapenem‑resistant Pseudomonas aeruginosa pneumonia?

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Carbapenem-Resistant Pseudomonas aeruginosa: Genetic Mechanisms

Carbapenem resistance in Pseudomonas aeruginosa pneumonia is primarily caused by decreased expression of the outer membrane porin OprD (causing resistance to both imipenem and meropenem), upregulation of efflux pumps, and acquisition of carbapenemase genes including metallo-β-lactamases (VIM, IMP, NDM, SPM) and Class A/D enzymes (GES, KPC, OXA-48-like). 1

Primary Resistance Mechanisms

1. OprD Porin Loss (Most Common)

The most frequent mechanism involves mutational inactivation or decreased expression of the OprD outer membrane porin channel, which serves as the entry point for carbapenems 1. This mechanism:

  • Can cause resistance to both imipenem and meropenem
  • Depending on the specific OprD alteration, may cause selective imipenem resistance while sparing other β-lactams
  • Was identified as the main mechanism in multiple studies of carbapenem-resistant P. aeruginosa 2

2. Efflux Pump Overexpression

P. aeruginosa possesses multiple efflux pumps that can be constitutively expressed or upregulated by mutation 1. These pumps mediate intrinsic resistance to many antimicrobials and contribute significantly to carbapenem resistance when overexpressed.

3. Carbapenemase Production

Metallo-β-lactamases (MBLs) - Class B:

  • VIM (most globally prevalent) 1, 3
  • IMP (first identified in Japan 1991, now worldwide) 1, 3
  • NDM (increasingly reported) 3, 4
  • SPM (particularly in South America) 5, 3

These enzymes are particularly concerning because they hydrolyze all β-lactams except aztreonam and cannot be inhibited by traditional β-lactamase inhibitors 6.

Class A Carbapenemases:

  • KPC (Klebsiella pneumoniae carbapenemase) 3, 4
  • GES 3, 4

Class D Carbapenemases:

  • OXA-48-like enzymes 3, 4

Geographic and Clonal Distribution

High-risk clones associated with carbapenem resistance include:

  • ST235 (most prevalent globally) 4, 7
  • ST111 2, 4
  • ST233, ST357, ST308, ST277 4

These sequence types are frequently associated with multidrug resistance and nosocomial spread 4, 7, 8.

Clinical Implications

Important Caveats:

  1. Not all carbapenem-resistant P. aeruginosa carry carbapenemases: Many isolates achieve resistance solely through OprD loss and/or efflux pump upregulation 1, 9. These "carbapenem-resistant but cephalosporin-susceptible" (CR/Ceph-S) strains may remain susceptible to antipseudomonal cephalosporins or piperacillin-tazobactam 9.

  2. Regional variation is substantial: The prevalence of specific carbapenemase genes varies dramatically by geography. For example, VIM predominates in Europe, IMP in Asia, and SPM in Brazil 3.

  3. Co-resistance patterns: Carbapenemase-producing strains frequently carry additional resistance determinants affecting fluoroquinolones, aminoglycosides, and other β-lactams 1, 10, 4, 7.

  4. Plasmid-mediated resistance is relatively uncommon: Only approximately 10% of carbapenemase-carrying genetic elements in P. aeruginosa are conjugative or mobilizable plasmids, suggesting clonal expansion plays a major role in dissemination 4.

Diagnostic Approach:

Rapid genotypic or phenotypic testing for carbapenemase production should be performed when carbapenem resistance is identified 6. This information directly impacts treatment selection, as:

  • MBL-producing strains remain susceptible to aztreonam (when combined with β-lactamase inhibitors)
  • Non-carbapenemase-producing resistant strains may respond to alternative β-lactams
  • Carbapenemase type influences choice of newer agents (ceftolozane-tazobactam, ceftazidime-avibactam, imipenem-relebactam, cefiderocol) 6, 11

The molecular characterization of resistance mechanisms is essential for optimizing therapy and implementing infection control measures to prevent spread of high-risk clones 6, 12, 11.

References

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