In human medicine, the best antibiotics for any compromised lung condition (bronchitis on top of asthma, etc...) are the Quinolones. The quinolones (cipro, levaquin in the human world) are very well tolerated and also very broad spectrum. Bean was on one for a skin infection. If Tiger hasn't been on a quinolone yet, that might be a good next choice? (up to the vet, of course, but a suggestion could never hurt).

**Freedom mentioned Baytril...I believe that is a quinolone.