Escherichia coli O157 : H7 becomes resistant to sodium chlorate in pure culture, but not in mixed culture or in vivo

Aims: Chlorate kills Escherichia coli O157:H7 and may be an effective feed additive for use in food animals. This study was designed to determine if d evelopment of chlorate-resistant E. coli O157:H7 strains was likely. Methods and Results: Escherichia coli O157:H7 was chlorate-sensitive, but b ecame chlorate-resistant in pure batch culture and in sterilized faecal flu id; it was killed in mixed culture and did not become resistant. Addition o f chlorate to continuous pure cultures caused chlorate resistance, but chlo rate addition to mixed continuous culture eliminated E. coli O157:H7 and no resistance occurred. Piglets challenged with E. coli O157:H7 were treated with chlorate; populations were reduced and colonies were always chlorate-s ensitive. Conclusions: Chlorate-resistant E. coli O157:H7 can be selected in pure, bu t not mixed culture, and results suggest that terminal chlorate feeding wil l not select for chlorate-resistance in vivo. Significance and Impact of the Study: Chlorate can reduce food-borne pathog ens prior to harvest, but development of resistance does not appear likely in vivo.