yes you are right. There is no test for bacteria in the inner ear. There is no also test for virus in the inner ear for us alived either. Why gentamicine injection for innere ear for menieres then? it is bacterial and antibiotic does not work for virus. At the end the ears are close to teeth and jaw. Imaging that  after the dental work you finish with tinnitus. The antibiotic does not work. And later the tinnitus progress into menieres int the same ear. Imaging that after the tooth extraction the infection can spread further in the bone ( sculp). If you have amalgams in mouth the banch of bacterias can be resistant ( research based info). Will the antibioitic help such antibioitic after the tooth extraction? The question is why for some the gentamicin injection help and some not if gentamicin is antibacterial? Maybe is good to look ate what bunch of bacterias aerobic and anaerobic are there in that bone/ear and what for is the gentamicin? katerina