What is the difference in bacterial coverage between ertapenem and meropenem?

Key Differences in Bacterial Coverage Between Ertapenem and Meropenem

The critical difference is that ertapenem lacks activity against Pseudomonas aeruginosa and Enterococcus species, while meropenem covers both of these organisms along with other non-fermentative gram-negative bacilli. 1

Spectrum Coverage Comparison

Organisms Covered by BOTH Carbapenems

• ESBL-producing Enterobacteriaceae (both agents are highly effective) 1
• AmpC-producing organisms 2, 3
• Gram-positive aerobes including methicillin-sensitive Staphylococcus aureus and penicillin-susceptible streptococci 2, 4
• Anaerobes including Bacteroides fragilis group, Clostridium perfringens, Peptostreptococcus species 2, 5
• Most Enterobacteriaceae (E. coli, Klebsiella, Proteus, Serratia, Citrobacter) 2, 4

Organisms Covered ONLY by Meropenem (Group 2 Carbapenem)

• Pseudomonas aeruginosa and other non-fermentative gram-negative bacilli 1, 6
• Enterococcus species 1, 2
• Other nosocomial non-fermenters commonly associated with hospital-acquired infections 3

Organisms NOT Covered by Either Agent

• Methicillin-resistant Staphylococcus aureus (MRSA) 2
• Enterococcus faecium (meropenem also lacks reliable activity) 2
• Stenotrophomonas maltophilia 2
• MDR-resistant gram-positive cocci 1

Clinical Classification and Implications

Ertapenem is classified as a Group 1 carbapenem with activity against ESBL-producing pathogens but NOT active against P. aeruginosa and Enterococcus species. 1

Meropenem is classified as a Group 2 carbapenem (along with imipenem and doripenem) which shares activity against non-fermentative gram-negative bacilli. 1

Practical Clinical Decision-Making

Use Ertapenem When:

• Community-acquired infections where Pseudomonas and Enterococcus are unlikely 3, 7
• Outpatient parenteral antimicrobial therapy (due to once-daily dosing from 4-hour half-life) 2, 3
• De-escalation therapy after clinical improvement in patients with susceptible Enterobacteriaceae 6
• Urinary tract infections (40-47% of comparative study populations) 6
• Community-acquired pneumonia, complicated intra-abdominal infections, acute pelvic infections in non-critically ill patients 8, 3

Use Meropenem When:

• Hospital-acquired or ventilator-associated pneumonia where Pseudomonas coverage is essential 6
• Severe nosocomial infections requiring broad gram-negative coverage 2
• Febrile neutropenia 6
• Bacterial meningitis (meropenem has lower seizure risk than imipenem and achieves excellent CSF penetration) 6
• Any infection where Pseudomonas aeruginosa or Enterococcus is suspected or documented 1
• Critically ill patients with polymicrobial or nosocomial infections 2

Important Caveats

Antimicrobial stewardship concerns: Both agents should be used judiciously to preserve carbapenem activity, as carbapenem-resistant K. pneumoniae has become a critical global issue. 1

Mortality equivalence in specific populations: In non-bacteremic infections and urinary tract infections caused by susceptible Enterobacteriaceae, ertapenem showed no significant mortality difference compared to meropenem/imipenem, though studies had moderate-to-high risk of bias due to selection of less severe cases for ertapenem. 6

Dosing differences: Ertapenem's extended 4-hour half-life allows once-daily dosing (1 gram daily), while meropenem's 1-hour half-life requires multiple daily doses. 2, 3

Resistance mechanisms affect both similarly: Both are susceptible to carbapenemases (KPC, NDM, VIM, IMP) and require intact outer membrane porins for activity. 2, 7