FIG 2.

Stx2a levels are diminished when cells are grown in supernatants from high-iron media. Cell-free supernatants of E. coli 0.1229ΔB17::FRT grown in various media were used to culture E. coli O157:H7 strain PA2. For comparison, PA2 was grown in fresh medium of the same composition, as indicated by “broth control.” Stx2a levels were measured by an R-ELISA and normalized to the optical density of each culture; the means and SEM are reported (n = 3 for all except 10 μM Fe cultures, in which n = 2). The metal chelators EDTA and 2,2′-bipyridyl (bipy) were added to media at 0.2 mM. Statistical significance was determined by two-way analysis of variance (ANOVA) and Sidak’s multiple-comparison test, assigning LB as the standard for the broth control and 0.1229ΔB17::FRT supernatant groups (***, P < 0.001; ****, P < 0.0001).