Enterococcus avium Sensitivity to Zosyn (Piperacillin/Tazobactam)
Zosyn (piperacillin/tazobactam) should NOT be relied upon for empirical coverage of Enterococcus avium, as this organism is intrinsically resistant to piperacillin/tazobactam and all beta-lactam/beta-lactamase inhibitor combinations.
Key Microbiological Considerations
Enterococcus avium is fundamentally different from E. faecalis and E. faecium in its antibiotic susceptibility profile. While piperacillin/tazobactam demonstrates excellent activity against E. faecalis (96% susceptibility) 1, this activity does NOT extend to E. avium, which belongs to a different enterococcal species group with intrinsic resistance mechanisms.
Why Piperacillin/Tazobactam Fails Against E. avium
- E. avium possesses intrinsic resistance to beta-lactams that cannot be overcome by beta-lactamase inhibitors like tazobactam, unlike beta-lactamase-producing E. faecalis where tazobactam restores activity 2
- The favorable enterococcal coverage cited in guidelines specifically refers to E. faecalis and E. faecium, not other Enterococcus species 3
- Even for susceptible enterococci, piperacillin/tazobactam requires high doses and may show treatment failure rates of 37-48% when resistant Gram-positive organisms are present 4
Recommended Empirical Alternatives for E. avium
If E. avium is suspected or confirmed, switch immediately to:
First-Line Options
- Linezolid 600 mg IV every 12 hours for serious infections including intra-abdominal infections and bloodstream infections 3
- Daptomycin 8-12 mg/kg IV once daily for bacteremic infections 3, 5
- Vancomycin 30 mg/kg per 24 hours IV in 2 divided doses (target trough 10-20 μg/mL) for most infections 3
For Uncomplicated Urinary Tract Infections Only
- Nitrofurantoin 100 mg PO four times daily for 3-7 days 3, 5
- Fosfomycin 3 g PO single dose or every other day 3
Critical Clinical Pitfalls
Do not assume all enterococci behave like E. faecalis. The guidelines emphasizing piperacillin/tazobactam for enterococcal coverage in intra-abdominal infections 3 are based on E. faecalis epidemiology, which represents 97% of enterococcal endocarditis cases 3. E. avium is a rare species with distinct resistance patterns.
Always obtain species-level identification and susceptibility testing when enterococci are isolated from sterile sites 3. The American Heart Association strongly recommends routine susceptibility testing for all enterococcal isolates to penicillin, vancomycin, and aminoglycosides 3.
Consider infectious disease consultation for any multidrug-resistant organism including atypical enterococcal species 3. E. avium infections are uncommon and may require specialized treatment approaches not covered in standard empirical regimens.
When Piperacillin/Tazobactam IS Appropriate for Enterococci
Piperacillin/tazobactam remains an excellent choice for empirical coverage of E. faecalis in:
- Severe community-acquired intra-abdominal infections 3
- Hospital-acquired intra-abdominal infections without critical illness 3
- Complicated intra-abdominal infections in high-risk adults 3
However, this does NOT apply to E. avium or other non-faecalis/non-faecium species.