Fig 4.

Effects of xanthine oxidase and hypoxanthine on bacterial growth and on Stx production in STEC. (A to C) Graphs of bacterial growth, measured as OD₆₀₀ values, in response to XO and various concentrations of hypoxanthine or other nucleosides. The x axis in panels A to C is the logarithm of the nucleoside or purine concentration, in moles/liter (M). (A) Growth inhibition in the presence of XO plus hypoxanthine, but not XO plus other nucleosides, on EPEC E2348/69. (B) Comparison of the susceptibilities of 3 strains of E. coli to growth inhibition by various concentrations of hypoxanthine in the presence of a fixed concentration of XO, 1 U/ml. (C) Inhibition of growth under anaerobic conditions in thioglycolate medium for three bacterial strains. (D to F) Effect of XO with or without hypoxanthine on Stx production from human STEC strain Popeye-1 (O157:H7, Stx2 only). (D) Although asterisks are omitted, Stx in the supernatant medium was significantly higher in the presence of XO than in its absence for all 3 concentrations of hypoxanthine tested. (E) Reversal of Stx induction by H₂O₂-neutralizing agents. A total of 1 U/ml XO and 400 μM hypoxanthine were used. Catalase (added to a final concentration of 600 U/ml) and glutathione (final concentration of 5 mM) reversed the inducing effect of hypoxanthine plus XO. *, significantly less Stx than with hypoxanthine plus XO. (F) Effect of varying the amount of XO in the presence of a fixed concentration of hypoxanthine. *, significant compared to the no-hypoxanthine control for each amount of XO. hypo, hypoxanthine.