Purpose: The purpose of this study was to evaluate the effect of sub-lethal chlorine stress on the virulence of STEC.
Methods: One strain of O157:H7 and 6 strains of non-O157 STEC (O26:H11, O103:H1, O104:H4, O111:NM, O121:NM, and O145:NM) were subjected to four consecutive 24h-exposures to chlorine stress (2 and 5 ppm). The surviving cells were selected as resistant strains. Cells without chlorine stress (control) and with chlorine stress were stored at -80°C until use. Real-time PCR was used to quantify stx1 gene in each strain. Lactate dehydrogenase assay was performed to determine cytotoxicity in Vero cells. Cytotoxicity levels (%) of cells were calculated and compared using Student’s t-test.
Results: Results showed that stressed O103:H1, O104:H4, and O145:NM had relatively higher (P < 0.05) gene expression of stx1 and cytotoxicity levels than the control, whereas stressed O157:H7 exhibited only higher cytotoxicity level than the control. For example the cytotoxicity levels (%) were 50.50 ± 5.41 vs. 67.99 ± 5.91 in control and cells of O103:H1 subjected to 2 ppm chlorine stress, respectively. Similar results were observed for O104:H4 (50.17 ± 5.60 vs. 67.21 ± 4.57, respectively) and O145:NM (50.51 ± 4.33 vs. 59.47 ± 2.31, respectively), indicating that chlorine stress induce increased virulence in the selected strains.
Significance: Data from this study suggest that multiple exposures of STEC strains to chlorine stress at sub-lethal concentration may enhance the gene expression of Shiga toxin and its extracellular production. The information will be useful for designing intervention strategies against STEC in food processing plants.