Is Pseudomonas aeruginosa susceptible to ertapenem if it is susceptible to meropenem (Merrem) and imipenem (Primaxin)?

Pseudomonas aeruginosa is NOT Susceptible to Ertapenem Despite Susceptibility to Meropenem and Imipenem

Pseudomonas aeruginosa is intrinsically resistant to ertapenem, even when susceptible to other carbapenems like meropenem and imipenem. This is a critical distinction in carbapenem susceptibility patterns that has important clinical implications for treatment decisions.

Carbapenem Susceptibility Patterns in Pseudomonas

• Carbapenems are divided into different groups with varying activity against Pseudomonas:

  • Group 1 carbapenems: Ertapenem - lacks reliable antipseudomonal activity
  • Group 2 carbapenems: Imipenem, meropenem, doripenem - have antipseudomonal activity 1

• In vitro antimicrobial susceptibility testing of Pseudomonas isolates has consistently shown:

  • Susceptibility to meropenem and imipenem
  • Resistance to ertapenem 2

Evidence Supporting Intrinsic Resistance

A comprehensive antimicrobial susceptibility study of bacterial isolates collected between 1978 and 2003 found that while Pseudomonas isolates were sensitive to meropenem and imipenem, the majority were resistant to ertapenem 2. This clearly demonstrates that susceptibility to one carbapenem does not predict susceptibility to all carbapenems.

The intrinsic resistance of Pseudomonas to ertapenem is due to several mechanisms:

• Multiple efflux pumps that may be expressed constitutively or upregulated by mutation 2
• Decreased expression of outer membrane porin channels (OprD) that can affect carbapenem entry 2
• Different binding affinities to penicillin-binding proteins (PBPs) 1

Clinical Implications

When treating Pseudomonas infections:

• Ertapenem should not be used for treatment of known or suspected Pseudomonas infections, regardless of susceptibility to other carbapenems 2
• For infections where Pseudomonas is a concern, appropriate carbapenem options include:
  • Meropenem (preferred in many cases due to dosing flexibility up to 6g daily) 2
  • Imipenem (with somewhat less activity against gram-negative bacilli than meropenem) 3
  • Doripenem (which may have more potent activity against P. aeruginosa than meropenem) 1

Carbapenem Stewardship Considerations

Interestingly, studies have shown that ertapenem use in hospitals does not negatively impact Pseudomonas susceptibility to other carbapenems:

• A 10-year study demonstrated that after ertapenem introduction to a hospital formulary, there was actually an improvement in Pseudomonas susceptibility to group 2 carbapenems 4
• Another study found that ertapenem adoption was associated with a downward trend in imipenem and ciprofloxacin use, which may have contributed to improved susceptibility of P. aeruginosa to imipenem 5

Conclusion

When interpreting susceptibility results for Pseudomonas aeruginosa, it's essential to recognize that susceptibility to meropenem and imipenem does not indicate susceptibility to ertapenem. Ertapenem should never be used for Pseudomonas infections, regardless of reported susceptibility to other carbapenems.