Updated: Aug 29, 2019
Recent test results indicate David does indeed have C. diff., which can be difficult to eradicate. Dr. Oh had David stop taking one of the antibiotics (Bactrim) because it won't help fight C. diff, and she said he will probably need to take the other antibiotic (Flagyl) for at least a month. Apparently, C. diff is a spore-forming bacterium and it can take a while for all the spores to "hatch." So you have to take antibiotics long enough to kill all the C. diff.
On the plus side, he is slowly reintroducing some fiber into his diet and his intestines feel better.