There are many other antibiotics that are effective against both gram negative and gram positive bacterial infections. Your vet should be able to recommend one. Do not take any dosing or medication advice from any one on any forum. We are not vets.
As Alex stated, more and more vets are veering away from Baytril as more and more bacteria are developing resistance to it.
While I understand that your vet wants to begin treatment now due to the lag in getting culture results, I am unlear as to why she just doesn't prescribe your animal an antibiotic that has a broad range of effectiveness while the culture is processed.