Acid adaptation and starvation effects on Shiga toxin production by Escherichia coli O157:H7.

Reverse transcription-polymerase chain reaction, enzyme-linked immunosorbent assay (ELISA), and gene fusion were used to determine differences in the expression of stx-II mRNA and the production of mature Stx protein following acid adaptation or starvation of Escherichia coli O157:H7 (ATCC 43895) and an isogenic rpoS mutant (FRIK 816-3) under static conditions and with shaking. The expression of stx-II mRNA in acid-adapted and starved cells was more extensive than that in nonstressed control cells. This effect was more pronounced for the rpoS mutant. Oxygenation (incubation with shaking) increased stx-II mRNA expression for both strains relative to the level of expression obtained with static conditions. ELISA results indicated that Stx production was enhanced more in the rpoS mutant than in its wild-type parent strain and that oxygenation enhanced Stx production for both strains but there were no detectable differences between stressed and nonstressed cells of either strain. The monitoring of the gene product of Stx-II alone with the use of stx-IIAB::lacZ gene fusions confirmed the induction of aeration and the absence of a stress effect for both the wild type and the rpoS mutant. These results indicate that oxygen enhances stx-II mRNA expression and Stx production in E. coli O157:H7. Stress conditions such as acid adaptation and starvation enhance stx-II toxin mRNA levels but do not enhance subsequent Stx toxin production.