Abstract

This study was designed to evaluate the antibiotic susceptibility, lactamase activity, gene expression, and protein profiles of antibiotic-sensitive Salmonella enterica serovar Typhimurium (S. Typhimurium^S) and antibiotic-resistant S. Typhimurium (S. Typhimurium^R) grown at pH 5.5 and 7.3. The antibiotic susceptibility and β-lactamase activity were measured by broth microdilution and nitrocefin assays, respectively. The relative gene expression of efflux pump- and outer membrane-associated genes was measured by real time - polymerase chain reaction (RT-PCR), and cell surface proteins were identified by two-dimensional gel electrophoresis (2-DE) analysis. S. Typhimurium^R grown at pH 7.3 shows the hyperproduction of β-lactamases regardless of antibiotic resistance. The expression of efflux pump-related genes (acrA, acrB and tolC) was mostly decreased and increased slightly in antibiotic-treated S. Typhimurium^R at pH 7.3. The identified proteins were classified into energy production, amino acid transport and metabolism, posttranslational modification, and carbohydrate transport and metabolism. The results suggest that the antibiotic resistance property of S. Typhimurium under mild acidic condition were highly associated with lactamase activity, efflux pump activity, and cell surface protein profiles. This study provides useful information for better understanding the mechanisms of β-lactam antibiotic-resistant S. Typhimurium exposed to acid stress.

Key words: Antibiotic resistance, β-lactamase, efflux pump, protein profile, two-dimensional gel electrophoresis (2-DE), Salmonella Typhimurium.