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A spontaneous one-step mutant of Salmonella paratyphi A selected on ampicillin showed cross-resistance to all beta-lactam antibiotics except imipenem and to aminoglycosides, chloramphenicol, tetracycline, trimethoprim, and quinolones. It also grew as small colonies. Examination of the cell envelope of the mutant showed a quantitative decrease in three major outer membrane proteins of 40.6, 39.6 (presumably porins), and 24 kilodaltons and quantitative as well as qualitative modifications in the ladder pattern of lipopolysaccharide. Direct evidence for decreased permeability in the mutant included reduced uptake of [3H]glucose and norfloxacin, reduced accessibility of aztreonam and benzylpenicillin to penicillin-binding proteins in whole cells, and decreased diffusion of lactose and cephaloridine into proteoliposomes that were reconstituted with outer membrane proteins from the mutant. There was also loss of invasiveness of the mutant into HeLa cells. We assume that a pleiotropic mutation was responsible for multiple alterations in the outer membrane components of the resistant mutant of S. paratyphi A.

Mutation of Salmonella paratyphi A conferring cross-resistance to several groups of antibiotics by decreased permeability and loss of invasiveness