I am unaware of any data supporting the addition of trimethoprim sulfa to ampicillin for this indication. In fact, the frequent recommendation to add gentamicin to ampicillin appears, to my knowledge, to be based only in vitro synergy data in the absence of confirmatory clinical data.

There has been a lot of misinformation in the literature for decades about the treatment of Listeria bacteremia and meningitis/encephalitis. Some of this arose based on publications of in vitro studies that claimed that "PCN and ampicillin are bacteriostatic against Listeria." This was based on dete...