Lack of synergism of ampicillin and gentamicin in experimental listeriosis.

Strain SLCC 4013 of Listeria monocytogenes is susceptible in vitro to ampicillin (MIC 0.5 mg/l) as well as to gentamicin (MIC 0.5 mg/l). Whereas treatment of mice infected with this virulent strain with 0.5 mg ampicillin twice a day resulted in a marked decrease in bacterial counts per spleen, the administration of 2 mg gentamicin twice a day hardly reduced bacterial multiplication. The combination of both drugs was not much more effective than ampicillin alone. Thus, a synergistic effect of both these antibiotics on intracellularly growing bacteria could not be demonstrated.