Use of a nonmedicated dietary supplement correlates with increased prevalence of streptomycin-sulfa-tetracycline-resistant Escherichia coli on a dairy farm

Abstract

We examined how a dietary supplement affects the prevalence of antibiotic-resistant Escherichia coli on a dairy farm in Washington State. Between 2001 and 2004 the prevalence of fecal E. coli strains resistant to streptomycin, sulfadiazine, and tetracycline (SSuT strains) declined from 59.2% to 26.1% in the calf population. In 2003 the dairy discontinued use of a dietary supplement, and we hypothesized that the decline in prevalence of SSuT strains was related to this change in management. To test this we established three treatments in which calves received no supplement, the dietary supplement with oxytetracycline, or the dietary supplement without oxytetracycline. Calves receiving either dietary supplement had a significantly higher prevalence of SSuT E. coli than the no-supplement control group (approximately 37% versus 20%, respectively; P = 0.03). Importantly, there was no evidence that oxytetracycline contributed to an increased prevalence of fecal SSuT E. coli. We compared the growth characteristics of SSuT and non-SSuT E. coli in LB broth enriched with either the complete dietary supplement or its individual constituents. Both the complete dietary supplement and its vitamin D component supported a significantly higher cell density of SSuT strains (P = 0.003 and P = 0.001, respectively). The dry milk and vitamin A components of the dietary supplement did not support different cell densities. These results were consistent with selection and maintenance of SSuT E. coli due to environmental components independent of antibiotic selection.

FIG. 1.

Frequency of antimicrobial drug resistance for all E. coli isolates shed from calves: year 2001 (n = 18 calves), year 2004 (n = 30), group receiving supplement without Pennox-50 (n = 9), group receiving supplement with Pennox-50 (n = 9), and group not receiving supplement or Pennox-50 (n = 9). + and *, statistically significant (P < 0.05 by Student's t test between years and repeated-measures ANOVA test for 2004 experiment, respectively); error bars indicate standard errors.

FIG. 2.

Frequency of antimicrobial drug resistance patterns for all E. coli isolates shed from calves: year 2001 (n = 18), year 2004 (n = 30), group receiving supplement without Pennox-50 (n = 9), group receiving supplement with Pennox-50 (n = 9), and group not receiving supplement or Pennox-50 (n = 9). + and *, statistically significant (P < 0.05 by Student's t test between years and repeated-measures ANOVA test for 2004 experiment, respectively). Resistance patterns are denoted by letters, where A is ampicillin, Ch is chloramphenicol, S is streptomycin, Su is sulfadiazine, and T is tetracycline. Susceptible isolates were susceptible to all above-mentioned antimicrobial drugs tested; error bars indicate standard errors.

FIG. 3.

Distribution of SSuT E. coli isolates over weeks of life for all the calves for year 2001 (n = 18) and year 2004 (n = 30); error bars indicate standard errors.

FIG. 4.

Absorbance values at 24 h of growth for SSuT and susceptible E. coli. LB, LB broth (six replicates for each resistotype); M/S, LB broth supplemented with dietary supplement (not containing oxytetracycline; three replicates for each resistotype); M/P, LB broth supplemented with milk powder (three replicates for each resistotype); Vit A, LB broth supplemented with vitamin A (three replicates for each resistotype); Vit D, LB broth supplemented with vitamin D (three replicates for each resistotype). Error bars indicate standard errors; *, statistically significant difference between SSuT and susceptible E. coli (P < 0.05 by Student's t test).