Chlorine treatment enhances expression of virulence genes in Shiga toxin-producing Escherichia coli (STEC). Quantitative RT-PCR of STEC was performed to determine the relative levels of stx1 (A), stx2 (B), and eae (C). Ct (threshold cycle) values of stx1, stx2, and eae were normalized to that of the housekeeping gene, mdh. The relative abundance of virulence genes in stressed cells was then calculated by normalizing to the expression level of corresponding genes in control cells. Data from three independent experiments are presented as means ± SE. *P < 0.05.