Which organisms are not covered by meropenem?

Meropenem Coverage Gaps

Meropenem does NOT cover methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant Staphylococcus epidermidis (MRSE), Enterococcus faecium, Stenotrophomonas maltophilia, and organisms producing metallo-β-lactamases (MBLs) such as NDM, VIM, and IMP.

Key Organisms NOT Covered by Meropenem

Gram-Positive Resistant Organisms

• MRSA and MRSE: Meropenem has no in vitro activity against methicillin-resistant staphylococci 1. These organisms require vancomycin or linezolid for coverage 2.
• Enterococcus faecium: Unlike E. faecalis (which meropenem covers when vancomycin-susceptible), E. faecium is uniformly resistant to meropenem 3, 4.

Gram-Negative Resistant Organisms

• Stenotrophomonas maltophilia: This organism produces chromosomal metallo-β-lactamases (carbapenemases) that hydrolyze meropenem 3. It remains intrinsically resistant to all carbapenems.

• Metallo-β-lactamase (MBL) producers: Organisms producing NDM, VIM, or IMP carbapenemases are resistant to meropenem 5, 6. These enzymes hydrolyze all β-lactams except aztreonam 6. For MBL-producing CRE, consider:

  • Cefiderocol as monotherapy 7
  • Aztreonam plus ceftazidime-avibactam combination 7

• Carbapenem-resistant Enterobacterales (CRE): While meropenem is a Group 2 carbapenem with activity against non-fermentative gram-negatives 5, CRE with carbapenemase production (especially MBLs) are resistant. The 2017 WSES guidelines note that carbapenems lack activity against "MDR-resistant gram-positive cocci" and carbapenem-resistant organisms 5.

Specific Gram-Negative Considerations

• Aeromonas hydrophila and Pseudomonas (Xanthomonas) maltophilia: These produce metalloenzymes capable of hydrolyzing carbapenems; strains with high enzyme expression are resistant to meropenem 8.

• Imipenem-resistant Pseudomonas aeruginosa: Cross-resistance occurs between meropenem and imipenem due to outer membrane protein alterations (loss of OprD porin). P. aeruginosa isolates that developed imipenem resistance during therapy show 2- to 32-fold reduced susceptibility to meropenem 8, 3.

Clinical Implications

When to Add Coverage

According to multiple guidelines 5, 9, 2:

• Add vancomycin or linezolid when MRSA is suspected (healthcare-associated infections, prior MRSA colonization, or >20% MRSA prevalence in your unit)
• Do NOT routinely cover enterococci in community-acquired intra-abdominal infections 9, but consider ampicillin addition in healthcare-associated infections 10
• For MBL-producing organisms, meropenem is ineffective—use cefiderocol or aztreonam-based combinations 6, 7

Important Caveat

While meropenem covers Enterococcus faecalis (vancomycin-susceptible isolates only) 1, empiric enterococcal coverage is not recommended for most community-acquired infections 9. The 2010 IDSA/SIS guidelines explicitly state: "Empiric coverage of Enterococcus is not necessary in patients with community-acquired intra-abdominal infection" 9.

Resistance Mechanisms to Remember

Meropenem resistance develops through 1:

1. Decreased outer membrane permeability (porin loss)
2. Reduced PBP affinity
3. Increased efflux pump expression
4. Production of carbapenemases or metallo-β-lactamases

The presence of a β-methyl group at C1 makes meropenem stable against renal dehydropeptidase-1, but this does NOT protect against bacterial carbapenemases 3.