Been diagnosed with a bacterial infection which has cause me you have acute diarrhea . put on antibiotics. when will the antibiotics start to work?

Depends: While the antibiotics may work to kills of the possible bacterial pathogens that caused your diarrhea, your diarrhea may take several days to weeks to greatly improve depending on the degree of "damage" done to the lining of your intestinal tract and intestinal flora(normal bacteria). This can take awhile to fully normalize, as in on the order of weeks.

Hopefully soon: If it truly is a bacterial infection (most diarrhea is from a viral infection) then you should start to feel better within the next couple days. But if things don't improve or if you get worse then see your doctor right away.