There is a small body of case studies that show that antibiotics can affect mood. Some research groups have shown a strong correlation with certain antibiotics and mood disorders in animal models. The literature is currently pretty scant on antibiotics and human mood, although there are some well-documented cases.
Azithromycin, for example, has induced toxic catatonia. Clarithromycin has brought on severe mania in a handfull of documented cases. Some antibiotics have been shown to lower MAO levels.
I think the interesting question is why do some antibiotics impact some people's affect? It's pretty fascinating when you think that there may be mircrobes in our nervous system much like those in our GI tract, right? Both good and bad, that get clobbered by antimicrobials. Or to think that the state of our GI tract can affect our mood, just like our mood can affect our GI tract...