Relapse following cessation of antibiotic therapy for mouse typhoid in resistant and susceptible mice infected with salmonellae of differing virulence.

Relapse after cessation of ampicillin therapy in inbred mice infected with salmonellae was studied. The incidence of relapse was influenced by the following. (A) The mouse strain, with gentically susceptible animals more likely to relapse than genetically resistant animals; a possible role for the Ity gene was observed. (B) The virulence of the infecting organism, with a more virulent strain causing relapse in both resistant and susceptible mice, a less virulent strain causing relapse in neither resistant nor susceptible mice, and an intermediate strain causing relapse in susceptible mice but not in resistant mice. (C) The duration of antibiotic therapy, with prolonged treatment preventing relapse seen after short-term therapy. In all cases, ampicillin failed to clear the infection, and the mice remained carriers.