Purpose: Therefore, the purpose of this study was to elucidate the effect of NaCl on cell morphology and gene expression of Escherichia coli O157:H7 and non-O157 strains.
Methods: E. coli O157:H7 NCCP11142 and non-O157 strains (E. coli O111 ATCC12795 and E. coli O26 ATCC43887) were exposed to 0%, 2%, and 4% NaCl at 35°C until bacterial populations reached stationary phase, and cell pellets from the culture were used to analyze gene expression levels of osmotic stress-related genes (betA, ompC, proV, and proW) and heat stress-related genes (clpB, dksA, dnaJ, dnaK, grpE, msbB, and rpoE) by quantitative real-time (qRT)-PCR. In addition, E. coli O157:H7 and non-O157 strains were attached on cover glasses in tryptic soy broth at 35°C for 24 h. The attached E. coli cells on the cover glasses were exposed to 0%, 2%, and 4% NaCl for 1 h, and cell morphology of E. coli strains was then observed under Field Emission Scanning Electron Microscopy (SEM).
Results: Of 11 tested genes, the levels of proV and proW (osmotic stress-related gene), and rpoE gene (heat stress-related gene) expressions were significantly increased (P < 0.05), as NaCl concentration increased, regardless of E. coli strain. The expression levels of proV were higher (P < 0.05) in E. coli O157:H7 than in non-O157 strains. In addition, SEM images showed that size of E. coli O157:H7 cells became smaller after exposure to NaCl, but cell sizes of non-O157 strains were not influenced by NaCl.
Significance: These results indicate that NaCl may affect stress-related gene expressions of E. coli O157:H7 and non-O157 strains and decrease the cell size of E. coli O157:H7.