It's likely the inflammation that's causing the depression rather than any particular molecule produced by a bacterium though. I have the same experience with UC to the extent that taking steroids causes almost immediate relief of the depressive symptoms and I relapse when I taper. In my last flare, I tried to track depressive symptoms against CRP levels in my blood but being depressed, lost interest... Joking aside, I also wasn't getting the blood tests with sufficient frequency for it to be useful. "The Inflamed Mind" by Edward Bullamore is a good read on the subject.

How can you be sure the steroids themselves aren't causing the mood change? I also have UC and steroids make me noticeably, behaviorally manic. I don't think the mania is due to the absence of inflammation.

I can't. But there's good evidence that inflammation is linked with depression and steroids unquestionably reduce inflammation. They may well have further effects. They are certainly not something I want to take consistently.

Same disease and same effect, Mood is often a bellweather of a flare up.