NaCl Influences Thermal Resistance and Cell Morphology of Escherichia coli Strains.

This study elucidated the effect of NaCl on the thermal resistance and cell morphology of Escherichia coli strains. E. coli O157:H7 NCCP11142, E. coli O111 ATCC12795 and E. coli O26 ATCC43887 were incubated in tryptic soy broth formulated with 0% NaCl (nonhabituated) as well as broth formulated with 2% and 4% NaCl (NaCl-habituated) at 35C. Nonhabituated, 2% NaCl-habituated and 4% NaCl-habituated E. coli O157:H7, E. coli O111 and E. coli O26 were heat-challenged at 50C. The transcriptomes (osmotic stress-related: betA, ompC, proV, proW and heat stress-related: clpB, dksA, dnaJ, dnaK, grpE, msbB, rpoE) were analyzed using real-time polymerase chain reaction and the morphology of E. coli strains was also observed by field emission scanning electron microscopy. The survival of E. coli O157:H7 was higher ( P < 0.05) than those of other strains after the heat challenge. The expression levels of proV, proW and rpoE increased ( P < 0.05) as NaCl concentration increased, but only proV expression levels were significantly different ( P < 0.05) among E. coli strains. The results indicate that the effect of NaCl on the thermal resistance and the cell morphology depends on the E. coli strain, and increased proV expression level by NaCl may cause the increased thermal resistance of E. coli O157:H7. Practical Applications NaCl is widely used as an antimicrobial and a preservative agent in food processing. However, Escherichia coli O157:H7 exposed to 2% and 4% NaCl showed thermal resistance, upregulated gene expression on osmotic- and heat-related genes and changed cell morphology. Thus, NaCl concentration should be limited in food not to increase E. coli O157:H7 tolerance to other stresses. [ABSTRACT FROM AUTHOR]