Q: Why Ampicillin remains the first line of treatment for listeriosis? 

Answer: Listeria is universally resistant to cephalosporins. Listeria also remains universally resistant to clindamycin. Trimethoprim/sulfamethoxazole (Bactrim) is the second line of treatment. Vancomycin can be used as a third line of therapy but although active in vitro, it does not have good penetration in vivo.