Zosyn (Piperacillin/Tazobactam) Will NOT Be Effective for Ampicillin-Resistant E. faecium UTI
No, Zosyn (piperacillin/tazobactam) will not work for ampicillin-resistant Enterococcus faecium in urine because E. faecium that is resistant to ampicillin demonstrates intrinsic resistance to all beta-lactam/beta-lactamase inhibitor combinations, including piperacillin/tazobactam. 1
Understanding the Resistance Pattern
Ampicillin-resistant E. faecium is classified as vancomycin-resistant enterococci (VRE) when it also demonstrates vancomycin resistance, defined as ampicillin and vancomycin-resistant enterococci with high-level resistance to aminoglycosides 1
The resistance mechanism in E. faecium is fundamentally different from other gram-negative organisms—it is NOT due to beta-lactamase production that tazobactam can inhibit, but rather due to altered penicillin-binding proteins that render all beta-lactams ineffective 2
While piperacillin/tazobactam demonstrates excellent activity against beta-lactamase-producing Enterobacteriaceae (E. coli, Klebsiella, Proteus), this activity does NOT extend to ampicillin-resistant enterococci 3
Recommended Treatment Options for Ampicillin-Resistant E. faecium UTI
For Lower UTI (Cystitis):
- Nitrofurantoin is a first-line oral option with intrinsic activity against VRE, including ampicillin-resistant strains 2
- Fosfomycin (3g single dose) has intrinsic activity against enterococci including VRE 2
- Doxycycline is another oral option with activity against VRE for uncomplicated cystitis 2
For Upper UTI (Pyelonephritis) or Bacteremic UTI:
- Linezolid should be used for confirmed or suspected upper and/or bacteremic VRE UTIs among ampicillin-resistant strains 2
- Daptomycin is reserved for confirmed or suspected upper and/or bacteremic VRE UTIs among ampicillin-resistant strains 2
- Ampicillin remains the drug of choice IF the organism is ampicillin-susceptible, but your isolate is resistant 2
Critical Clinical Pearls
Always verify susceptibility testing before initiating therapy—the distinction between ampicillin-susceptible and ampicillin-resistant E. faecium is crucial for treatment selection 2
Differentiate between colonization, asymptomatic bacteriuria, and true infection—unnecessary antibiotic use in patients simply colonized with VRE has become a significant problem in hospitals and long-term care facilities 2
Duration of therapy: Complicated UTI requires 10-14 days of therapy to ensure adequate source control and prevent relapse 4
Common Pitfalls to Avoid
Do not assume that beta-lactam/beta-lactamase inhibitor combinations work for ampicillin-resistant enterococci—this is a fundamental misunderstanding of the resistance mechanism 2
Avoid using quinupristin-dalfopristin or tigecycline as first-line agents due to concerns of toxicity, resistance, and insufficient supportive data—these should be evaluated case-by-case 2
Do not use aminoglycosides as monotherapy except for simple cystitis; they require combination therapy for complicated UTI 4
When to Escalate Care
Infectious disease consultation is highly recommended in the management of infections caused by multidrug-resistant organisms like ampicillin-resistant E. faecium 1
If clinical response is not evident within 48-72 hours of appropriate therapy, obtain repeat urine culture to guide targeted therapy 5, 4