Came here to mention AS and the klebsiella hypothesis. Like you said, nothing conclusive has come out of it (and certainly no evidence of using antibacterial treatment successfully to treat AS), but I still find it super interesting considering the high comorbidity between AS and intestinal diseases like Crohn's and ulcerative colitis. IIRC some of the more recent "AS is a form of reactive arthritis" truthers have focused on gut infections specifically as the pathway.

Big grain of salt, of course. Thankfully even if we can't pinpoint its exact etiology we still have a million proven drugs to throw at this thing.