I’ve often asked by medical students what additional coverage is gained with using piperacillin-tazobactam (Zosyn) over ampicillin-sulbactam (Unasyn).  The question comes up because so many people reflexively reach for “vanc-zosyn” for broad coverage.  The answer is Pseudomonas.

The more difficult question is what additional coverage is gained with ampicillin-sulbactam over piperacillin-tazobactam.  The answer is acinetobacter.

The additional coverage comes from the the sulbactam portion.  This isn’t to say that piperacillin-tazobactam should be a first line treatment, but it’s interesting nonetheless.