Does imipenem (carbapenem antibiotic) cover Enterococcus faecium in an elderly, immunocompromised, or medically complex patient?

Imipenem Does NOT Provide Reliable Coverage for Enterococcus faecium

Imipenem should not be relied upon for treating Enterococcus faecium infections, as this organism demonstrates intrinsic resistance to carbapenems, including imipenem. 1

Key Microbiologic Distinctions

E. faecium vs. E. faecalis

• E. faecium is intrinsically resistant to imipenem and other carbapenems (meropenem, doripenem), along with cephalosporins, cephamycins, aztreonam, and antistaphylococcal penicillins 1
• E. faecalis has "some activity" with imipenem, but E. faecium does not share this susceptibility 1
• Carbapenems as a class lack activity against E. faecium 2

Clinical Implications

• Ampicillin resistance is common in E. faecium (MICs usually ≥16 µg/mL), making it significantly more resistant than E. faecalis (MICs 2-4 µg/mL) 1
• Up to 95% of E. faecium strains are multidrug-resistant, compared to only 3% of E. faecalis 3
• Prior imipenem use has been associated with selection and emergence of ampicillin-resistant E. faecium 4

Recommended Treatment for E. faecium

First-Line Options

• Ampicillin (if susceptible, though uncommon): 12 g/24h IV in 6 divided doses for 4-6 weeks for endocarditis 1
• Vancomycin: 30 mg/kg/24h IV in 2 divided doses for 6 weeks (for penicillin-resistant strains) 1, 3

For Vancomycin-Resistant E. faecium (VRE)

• Linezolid: 600 mg IV/PO every 12 hours (strongly recommended for VRE) 3, 5
• Daptomycin: 8-12 mg/kg/day for serious infections 3
• Tigecycline: 100 mg IV loading dose, then 50 mg IV every 12 hours for polymicrobial infections including VRE 5

Site-Specific Alternatives

• Uncomplicated UTI: Fosfomycin 3g PO single dose or nitrofurantoin 100 mg PO every 6 hours 3, 5

Critical Clinical Pitfalls

• Never assume carbapenem coverage for enterococcal infections, particularly E. faecium 1, 2
• Differentiate between E. faecalis (which may have some imipenem activity) and E. faecium (which does not) through species identification 3, 6, 7
• Distinguish colonization from true infection before initiating anti-enterococcal therapy 3, 5
• Empiric enterococcal coverage is not routinely recommended for community-acquired intra-abdominal infections unless specific risk factors exist 1, 5
• Always obtain susceptibility testing for enterococcal isolates, particularly testing for high-level aminoglycoside resistance and vancomycin resistance 1