Previous research has suggested that nitrate-respiring pathogens such as Escherichia coli O157:H7 and Salmonella spp. are susceptible to chlorate salts due to the conversion of chlorate to chlorite by respiratory nitrate reductase. This study was conducted to determine the effect of chlorate on E. coli O157:H7 growth and chlorate biotransformation and to determine whether chlorite is produced in anaerobic culture of E. coli O157:H7. Final concentrations of E. coli O157:H7 were generally decreased by about 2 log units in incubations containing > or =5 mM chlorate, except when bacteria were pretreated with 10 mM chlorate. [(36)Cl]Chlorate metabolism by pure cultures of E. coli O157:H7 was not measurable above chlorate concentrations of 5 mM, but measurable chlorate reduction occurred in cultures containing 0.5, 1, or 5 mM [(36)Cl]chlorate. Pretreatment of E. coli O157:H7 with 5 mM nitrate did not increase the rate of chlorate conversion to chloride, suggesting that nitrate did not induce nitrate reductase isoforms capable of metabolizing chlorate in E. coli O157:H7. Pure cultures of E. coli O157:H7 preconditioned with 10 mM chlorate had an attenuated ability to transform [(36)Cl]chlorate to [(36)Cl]chloride with measurable chlorate reduction only occurring in 0.5 mM chlorate treatments. The hypothesis that E. coli O157:H7 is sensitive to chlorate by virtue of the reduction of chlorate to chlorite ion (ClO(2)(-)) was supported, but not proven, by the direct measurement of low concentrations of [(36)Cl]ClO(2)(-) in incubation media containing 0.5 mM [(36)Cl]ClO(3)(-). Collectively these results indicate that growth of E. coli O157:H7 in pure culture will be reduced in the presence of 5 mM or greater concentrations of sodium chlorate and that E. coli O157:H7 is capable of producing chlorite ions during the metabolism of chlorate.