Zosyn/Piperacillin is an old school antibiotic that used to be popular back in the "olden" times. Last time I had it prescribed to one of my animals it was used in conjunction with either gentamicin or amikacin (I can't remember which) but I do remember having to administer fluids in conjunction with the treatment.
I've actually seen it prescribed recently for an RI that didn't respond to Baytril in a friend's snake. I'm pretty sure it was combined with amikacin.
One of the drawbacks of an antibiotic becoming popular is that knuckleheads improperly inject their snakes or do not complete their dosing regimens - resulting in widespread resistance to a drug. This is what appears to be happenings with Baytril. We are seeing more and more people post that it isn't working and we are seeing more and more people tell us that their vets are going with alternative antibiotics.