Effect of trisodium phosphate adaptation on changes in membrane lipid composition, verotoxin secretion, and acid resistance of Escherichia coli O157:H7 in simulated gastric fluid

Abstract

Escherichia coli O157:H7 (HEC), E. coli O157:H7 rpoS mutant (HEC-RM), and nonpathogenic E. coli (NPEC) were step-wise adapted to trisodium phosphate (TSP) by incubation in broths of increasing concentration, from 0% to 0.6%, at 37 °C for 24 h. After incubation at each concentration, each population was examined for acid resistance ( D value) in simulated gastric fluid of pH 1.5, cell envelope membrane lipid composition, and intracellular and extracellular verotoxin concentrations. The ratio of cis-vaccenic acid (18:1ω7c) to palmitic acid (16:0) increased, indicating increased membrane fluidity with increasing TSP concentration up to 0.4%, but decreased at 0.6%. HEC and HEC-RM adapted at 0.4% TSP had the highest verotoxin concentrations of 1805 and 1879 ng/ml, respectively. In addition, with HEC the ratio of extracellular to intracellular verotoxin concentration decreased at higher TSP concentrations. In contrast, the ratio for HEC-RM increased at 0.4% TSP. HEC adapted to 0.4% TSP had the greatest survival in gastric fluid (58 min D value) among all treatments. For HEC, the increase in membrane fluidity was associated with increased acid resistance and extracellular verotoxin concentration for cells adapted to 0.4% TSP. In contrast, the increase in membrane fluidity was associated with decreased acid resistance of TSP adapted HEC-RM although the extracellular verotoxin concentration increased. Therefore, the deletion of the rpoS gene appeared to affect the changes in verotoxin concentration and acid resistance of TSP adapted E. coli O157:H7.