Zosyn

Piperacillin is an extended-spectrum penicillin (ureidopenicillin) that inhibits bacterial cell wall synthesis by binding penicillin-binding proteins (PBPs), disrupting peptidoglycan cross-linking and causing cell lysis. Tazobactam is a beta-lactamase inhibitor that irreversibly binds and inactivates many bacterial beta-lactamases, protecting piperacillin from enzymatic degradation and extending its spectrum to beta-lactamase-producing organisms.

Spectrum of activity: Gram-positive cocci (streptococci, methicillin-sensitive Staphylococcus aureus—not MRSA), gram-negative rods including Pseudomonas aeruginosa, Enterobacteriaceae, and anaerobes (e.g., Bacteroides fragilis). It does NOT cover MRSA, atypicals, or most ESBL/carbapenemase-producing organisms reliably.

Clinical uses: Empiric therapy for hospital-acquired/nosocomial infections, febrile neutropenia, intra-abdominal infections, polymicrobial infections (e.g., diabetic foot infections, necrotizing fasciitis), aspiration pneumonia, and healthcare-associated pneumonia—often combined with vancomycin to cover MRSA and sometimes an aminoglycoside/fluoroquinolone for additional Pseudomonas coverage.

Adverse effects: Hypersensitivity reactions (cross-reactivity with other penicillins/cephalosporins), GI upset, interstitial nephritis, drug-induced thrombocytopenia, and notably can cause a false elevation in galactomannan assay (used for Aspergillus detection) and can lead to hypokalemia due to its sodium salt formulation with obligate renal potassium wasting.

Important clinical pearls: Zosyn plus vancomycin has been associated with increased risk of acute kidney injury (AKI) compared to other combinations—a key point tested on Step 2. Dose adjustment required in renal impairment. Need to distinguish from true penicillin allergy history (severe anaphylaxis vs. mild rash) when considering use.