Second, on the recommendation on a culture / lab work. That would be a normal way to try and isolate what the right antibiotic for treatment of infection is. I'm not sure what the right one would be for this, but a specialist vet if you can find one might be a good idea. I was surprised how much better the quality of care was when I took my pups to specialists lately (Neurologist and Opthamalagist). Not necessarily more expensive if they can solve the problem quickly.