Drug of choice for listeria monocytogenes is:

Ampicillin REF: Jawett's 24th edition Section III. Bacteriology > Chapter 13. Non-Spore-Forming Gram-Positive Bacilli: Corynebacterium, Propionibacterium, Listeria, Erysipelothrix, Actinomycetes, & Related Pathogens Many antimicrobial drugs inhibit listeria in vitro. Clinical cures have been obtained with Ampicillin with erythromycin, or with intravenous trimethoprim-sulfamethoxazole. Cephalosporins and fluoroquinolones are not active against L monocytogenes. Ampicillin plus gentamicin is often recommended for therapy, but gentamicin does not enter host cells and may not help treat the listeria infection