What is metallo‑β‑lactamase (MBL) and how does it explain the extensive β‑lactam resistance in a multidrug‑resistant Pseudomonas aeruginosa isolate?

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Last updated: November 6, 2026 • View editorial policy

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What is Metallo-β-Lactamase (MBL)?

Metallo-β-lactamases (MBLs) are Class B carbapenemase enzymes that hydrolyze virtually all β-lactam antibiotics except aztreonam and cannot be inhibited by traditional serine β-lactamase inhibitors like clavulanate, tazobactam, or avibactam 1.

Mechanism and Clinical Significance

MBLs represent one of the most concerning resistance mechanisms in multidrug-resistant Pseudomonas aeruginosa and other Gram-negative bacteria because they:

  • Destroy all β-lactams except monobactams (aztreonam) through hydrolysis 1
  • Require zinc ions at their active site (hence "metallo") rather than serine residues like other β-lactamases 2
  • Resist inhibition by conventional β-lactamase inhibitors (clavulanate, sulbactam, tazobactam, avibactam, vaborbactam, relebactam) 1
  • Often co-exist with extended-spectrum β-lactamases (ESBLs), creating organisms resistant to essentially all β-lactams except aztreonam 1

Major MBL Types

The clinically important MBL families include 1, 3:

  • VIM (Verona integron-encoded metallo-β-lactamase) - most common globally
  • IMP (Imipenemase) - particularly prevalent in Asia-Pacific regions
  • NDM (New Delhi metallo-β-lactamase) - rapidly emerging worldwide
  • SPM (São Paulo metallo-β-lactamase) - primarily in South America
  • GIM (Germany imipenemase) - rare
  • FIM (Florence imipenemase) - rare

Why MBLs Explain Extensive β-Lactam Resistance in P. aeruginosa

In multidrug-resistant P. aeruginosa, MBL production creates extensive β-lactam resistance through:

  1. Broad substrate spectrum: MBLs hydrolyze penicillins, cephalosporins (including anti-pseudomonal agents like ceftazidime and cefepime), and carbapenems (imipenem, meropenem) 2, 3

  2. Genetic mobility: MBL genes are typically located on mobile genetic elements (integrons, transposons, plasmids) that facilitate horizontal transfer between bacteria and accumulate multiple resistance determinants 3, 4

  3. Synergy with intrinsic resistance: P. aeruginosa already possesses intrinsic resistance mechanisms (efflux pumps, AmpC β-lactamase, low outer membrane permeability). When MBL production is added, the organism becomes resistant to virtually all β-lactams except aztreonam 2

  4. Co-resistance patterns: MBL-producing isolates frequently carry genes conferring resistance to aminoglycosides, fluoroquinolones, and other antibiotic classes, resulting in extensively drug-resistant (XDR) or difficult-to-treat resistant (DTR) phenotypes 3, 5

Clinical Implications

MBL-producing P. aeruginosa infections are associated with:

  • Higher mortality rates and worse clinical outcomes 5
  • Faster progression to death compared to non-MBL producers 5
  • Limited treatment options - primarily cefiderocol, aztreonam-based combinations (when aztreonam susceptibility is preserved), or colistin-based regimens 1
  • ICU stay and invasive devices as major risk factors 5

Diagnostic Considerations

Rapid identification of MBL production is crucial because it fundamentally changes treatment strategy 1. Standard carbapenem resistance testing alone is insufficient - specific carbapenemase family identification (distinguishing MBL from KPC or OXA-48-like enzymes) is essential because:

  • MBL producers remain susceptible to aztreonam (unlike KPC producers)
  • Traditional β-lactam/β-lactamase inhibitor combinations (ceftazidime/avibactam, meropenem/vaborbactam) are ineffective against MBLs
  • Time to appropriate therapy directly impacts mortality 1

Common pitfall: Assuming all carbapenem-resistant P. aeruginosa can be treated with newer β-lactam/β-lactamase inhibitor combinations. This is incorrect for MBL producers, which require aztreonam-based regimens or cefiderocol 1.

References

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