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.