Effects of Salt Stress on the Antimicrobial Drug Resistance and Protein Pattern of Staphylococcus aureus

Background: Staphylococcus aureus is the causative agent of a high percentage of nosocomially acquired infections and food-borne illnesses. Antimicrobial resistance of S. aureus, especially methicillin-resistant S. aureus (MRSA), continues to be a concernfor clinicians worldwide. Objectives: The aim of this study was to investigate the effects of salt stress on the antimicrobial drug resistance and protein profile of S. aureus. Materials and Methods: Staphylococcus aureus (ATCC 25823) was grown in trypticase soy broth at 37°C. Cells in the exponential growth phase were gradually exposed to sub-lethal salt stress with concentrations ranging from 5% to35% (wt/vol). There after, these cells were harvested and re-suspended in a tube containing 0.5mL of saline. To standardize the number of bacteria, the bacterial suspension was compared to the 0.5 McFarland standard suspension.Antibiotic susceptibility was determined using the disk diffusion method, and the method involved plating of cell suspensions with stressed cells and unstressed cells on Mueller-Hinton agar plates. The pooled proteins from each condition were analyzed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Results: Compared to the unstressed cells, the cells exposed to salt showed significant changes in resistance to rifampicin (P=0.032), penicillin (P=0.02) and methicillin (P=0.001). Furthermore, SDS-PAGE analysis of pooled proteins from cells exposed to salt showed changes in the protein profile. Conclusions: We conclude that salt stress is responsible for the changes in protein profileand antimicrobial resistance of S. aureus, especially to methicillin