Permeability and penicillin-binding protein alterations in Salmonella muenchen: stepwise resistance acquired during beta-lactam therapy

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Abstract
A patient with Salmonella muenchen sepsis was unsuccessfully treated with amipicillin. During therapy, four strains that showed stepwise ampicillin resistance and affected other .beta.-lactams and unrelated antibiotics were isolated sequentially. Resistance was caused by decreased outer membrane permeability associated with diminished expression of porin OmpF. Furthermore, the most resistant isolate overproduced the PBP 3 target molecule.