Interpretive Summary: Chlorate kills E. coli O157:H7. Recently, chlorate has been proposed to be a beneficial feed additive for cattle. However, mutants of E. coli resistant to chlorate have been isolated for many years in the laboratory. This study examined the risk of selecting for a population that was highly-chlorate resistant in the animal gastrointestinal tract. Results indicated that: 1) E. coli O157:H7 was able to become chlorate resistant in the test tube, 2) chlorate resistance was an "expensive" trait that was not maintained without selective pressure, 3) chlorate-resistant mutants were not able to compete against the native population of gastrointestinal bacteria from cattle or in pigs. These results indicate that chlorate addition to cattle feed is not likely to select for a chlorate-resistant population. Therefore, the issue of resistance transfer is not likely to be a problem in the feedlots.
Technical Abstract: Sodium chlorate kills Escherichia coli O157:H7 and has been proposed as an additive for ruminant diets prior to slaughter. Chlorate kills E. coli through a non-specific reduction of chlorate by nitrate reductase. Cells lacking nitrate reductase are chlorate-resistant, and are easily selected in pure culture. The objective of this study was to determine if chlorate-resistant E. coli O157:H7 strains are selected in in vivo conditions. When E. coli O157:H7 was incubated in sterilized fecal juice with 10 mM chlorate, E. coli O157:H7 populations declined from 10**6 to 10**3 in 6 h (P < 0.05), but returned to 10**5 by 24 h and these cells were chlorate-resistant. E. coli O157:H7 incubated in fresh fecal fluid with 10 mM chlorate declined from 10**6 to 10**1 CFU/ml in 6 h (P < 0.05) and to 10**0 by 24 h (P < 0.05) and these colonies were chlorate- sensitive. Continuous culture of pure E. coli O157:H7 had a stable population of 10**7 CFU/ml that was always initially chlorate-sensitive (P < 0.05). Addition of 10 mM chlorate caused a transient decrease in CFU but colonies (n=20) were highly chlorate-resistant (P > 0.1). Continuous culture of E. coli O157:H7 in fresh fecal juice had a total anaerobic population of 10**10 cells/ml and E. coli 0157:H7 of 10**6 CFU/ml at t=0. When 10 mM chlorate was added, the E. coli 0157:H7 populations declined to 10**3(P < 0.05) by 60 h and no colonies (n=20) were chlorate-resistant. When weaned piglets (n=30) were challenged with E. coli O157:H7 and treated with chlorate, no isolates from the ileum, cecum, colon or rectum were chlorate-resistant. These results indicate that chlorate-resistant strains can be selected in pure, but not mixed culture. Suggesting that terminal chlorate feeding should not select for resistant strains in vivo.