Targeted microarray analysis of stress response and virulence gene expression in Escherichia coli O157:H7

Escherichia coli O157:H7 is a pernicious organism that remains a threat to humans through foodborne transmission. Targeted microarray analysis (n-125) was used to examine stress and virulence transcriptome changes in cells subjected to sublethal stresses. It was shown that logarithmic cells exposed to hydrogen peroxide (H2O2) and paraquat were more sensitive to oxidative stress than stationary phase cells. Similar transcriptional profiles were obtained for all logarithmic treatments, regardless of oxidant. Stx2a and stx2b were induced in all logarithmic treatments, with stx1, ler, 1pfA and a putative enterotoxin induced by 2.5 mM H2O2. In nutrient deprived cells, 24 genes were significantly affected. Regulators at all hierarchical levels were induced, including global, specific stress and small regulon regulators. Induction of ler-independent virulence genes was observed. Comparison of the nutrient deprived transcriptome to that of nutrient replenished cells revealed significantly different transcriptional profiles, with 55 genes differentially expressed. Seven virulence genes and all queried regulators were differentially expressed. Stationary phase cells were also exposed to acidified tryptic soy broth (TSB; pH 3.5), cold TSB (7.5°C), and 37°C TSB (pH 7.0). In TSB-treated cells, 72 genes were significantly affected, with 65 induced. All major global and specific stress response regulators were induced whilst rpoS was repressed. Genes related to replication and metabolism were induced, as were 12 virulence genes. Comparison of stationary phase acid-treated cells and reference cells produced 25 significantly affected genes. No induction of major acid response (AR) systems was observed, though coping mechanisms related to mild acid stress were recruited. Comparison of TSB acid-treated cells to TSB-treated cells resulted in 24 differentially expressed genes, with 19 down-regulated. Induction of AR 1 and 2 was observed. In cold shocked cells, 17 genes were significantly affected, with 10 induced. sigma S, the heat shock reguIon, AR 2, and stx1a were induced by cold shock, whilst transcript levels of ompC, ompF and housekeeping genes (gapA, tufA) were repressed. In conclusion, exposure to sublethal stress events lead to the induction of diverse stress response regulons, and virulence genes, which is of significant consequence to the food chain.