Perhaps they were soaking her in antibiotic for that decubitus ulcer they found on her back side a while back! That sore is further proof of the neglect Terri has been given at this facility. Terri deserves much better care than she's been given!
Those AB instillations are done locally in bed. The amount of AB you would have to put in a tub to reach therapeutic concentration would cost a fortune. Also, the rest of the body would be soaked as well, possibly inducing further problems.