Antimicrobial susceptibility of Salmonella typhimurium carrying the outer membrane permeability mutation SS-B

The antibiotic susceptibility profile of Salmonella typhimurium SS-B, a mutant susceptible to some antimicrobial agents, was studied in detail. Twenty-eight agents were tested, and eleven of these had MICs significantly lower (32- to greater than 250-fold) for the SS-B strain than for its parent. The drugs were generally hydrophobic or amphiphilic. Polymyxin B nonapeptide, which has a known outer membrane permeabilizing action, further reduced the MIC of several of these agents for the SS-B strain by a factor of approximately 10 to 30. In most cases, the resulting MICs were lower than the corresponding MICs for the parent strain grown in the presence of polymyxin B nonapeptide. In addition, the hydrophobic fluorescent probe N-phenyl naphthylamine was rapidly embedded in the membranes of the SS-B strain but was poorly embedded in those of the parent strain.