Efficacy of ampicillin therapy in experimental listeriosis in mice with impaired T-cell-mediated immune response.

The importance of intact host defense mechanisms for successful antimicrobial therapy was investigated via a comparison of the activities of ampicillin against experimental Listeria monocytogenes infections in normal mice and congenitally athymic (nude) mice. Nude mice were used for these experiments because recovery from infection with this organism depends on development of cellular immunity induced specifically by a T-cell-mediated reaction. When infections ampicillin per mouse (32 doses of 25 mg each), which is twenty times the dose required for a cure of infections in normal mice (8 doses of 5 mg each), would not cure infections in nude mice. With a reduction in inoculum to 10(5) colony-forming units, cures were obtained with a total ampicillin dose of 800 mg (32 doses of 25 mg each), but not with 400 mg (16 doses of 25 mg each). These studies show clearly that the efficacy of ampicillin against infections with L. monocytogenes is dependent upon intact host defense mechanisms.