Investigating mechanisms of lethality enhancement when Escherichia coli O157:H7, and selected E. coli mutants, were exposed to tert-butylhydroquinone (TBHQ) during ultra-high pressure (UHP) treatment. Escherichia coli O157:H7 EDL-933, and 14 E. coli K12 strains with mutations in selected genes, were treated with dimethyl sulfoxide solution of TBHQ (15-30 ppm), and processed with UHP (400 MPa, 23 +/- 2 degrees C for 5 min). Treatment of wild-type E. coli strains with UHP alone inactivated 2.4-3.7 log CFU ml(-1), whereas presence of TBHQ increased UHP lethality by 1.1-6.2 log CFU ml(-1); TBHQ without pressure was minimally lethal (0-0.6 log reduction). Response of E. coli K12 mutants to these treatments suggests that iron-sulfur cluster-containing proteins ([Fe-S]-proteins), particularly those related to the sulfur mobilization (SUF system), nitrate metabolism, and intracellular redox potential, are critical to the UHP-TBHQ synergy against E. coli. Mutations in genes maintaining redox homeostasis and anaerobic metabolism were associated with UHP-TBHQ resistance. The redox cycling activity of cellular [Fe-S]-proteins may oxidize TBHQ, potentially leading to the generation of bactericidal reactive oxygen species. A mechanism is proposed for the enhanced lethality of UHP by TBHQ against E. coli O157:H7. The results may benefit food processors using UHP-based preservation, and biologists interested in piezophilic micro-organisms.