Protective effect of exopolysaccharide colanic acid of Escherichia coli O157:H7 to osmotic and oxidative stress

Many strains of Escherichia coli O157:H7 produce, under stress, an exopolysaccharide (EPS) comprised of colanic acid (CA) and form mucoid colonies on minimal glucose agar (MGA) at ambient temperature. Previous research conducted in our laboratory involving a CA-proficient (W6-13) and a CA-deficient (M4020; wcaD∷Ekanr) strain of E. coli O157:H7 revealed that CA conferred acid and heat tolerance to E. coli O157:H7. Cells covered with CA were more persistent during acid (pH 4.5, 5.5, and 6.5) and heat (55 and 60°C) treatment. The goal of this research was to study the effect of CA on the fate of E. coli O157:H7 under osmotic and oxidative stress. Cells of W6-13 and M4020 were exposed to various concentrations of NaCl (0.5, 1.5, and 2.5 M) and H2O2 (0, 10, and 20 mM) in minimal glucose broth (MGB) at 22°C. Viable counts of E. coli O157:H7 were determined within 48 h of the osmotic stress and 3 h of the oxidative stress. The results suggest that cells of E. coli O157:H7 deficient in CA production are more susceptible than its wild-type parent to NaCl (P<0.05) and H2O2 (P≤0.05). This indicates that CA plays a role in protecting E. coli O157:H7 from osmotic and oxidative stress.