Environmental stresses alter Escherichia coli O157:H7 membrane lipid composition and verotoxin secretion

The influence of heat, pH, and organic acid stress adaptation on changes in membrane lipid composition, verotoxin concentration, and bacterial resistance to lethal conditions such as high temperature (57°C) or simulated gastric fluid (SGF, pH 1.5) were determined for Escherichia coli O157:H7 (HEC, ATCC 43895), an rpoS mutant of ATCC 43895 (HEC-RM, FRIK 816-3), and nonpathogenic E. coli (NPEC, ATCC 25922). D57°C values of heat adapted cells were up to 3.9 min greater than control cells for all three strains. D values in SGF of acid adapted cells with HCl were greater than nonadapted cells and alkali adapted cells with NaOH. Citric and lactic acid adapted HEC cells exhibited greater D values than non-adapted cells in SGF. Heat adaptation increased the amounts of palmitic acid (C16:0) and cis-vaccenic acid (C18:1o7c), which increased membrane fluidity. Regardless of acidulants, acid adaptation increased palmitic acid content and decreased cis -vaccenic acid content, which decreased membrane fluidity. Cells adapted to moderate stress conditions had less total verotoxin concentration than non-adapted cells; however, extracellular verotoxin concentration increased during heat and alkali adaptation, probably due to increased membrane fluidity. Although the rpoS gene did not influence specific membrane lipid composition changes, it did affect membrane fluidity patterns and verotoxin concentration changes. Results suggest that membrane fluidity has an important role in regulating bacterial resistance to lethal environments and verotoxin secretion.