Antimicrobial susceptibility of Escherichia coli isolates from dairy farms using organic versus conventional production methods.
OBJECTIVE
To compare antimicrobial susceptibility patterns of Escherichia coli isolates cultured from fecal samples from cows and calves on dairy farms that used organic (ie, no or severely limited antimicrobial use) versus conventional production methods.


METHODS
Cross-sectional study.


METHODS
Fecal samples from 10 cows and 10 calves on each of 30 organic dairy farms and 30 neighboring conventional dairy farms in Wisconsin.


METHODS
E. coli isolates obtained from the fecal samples were tested for susceptibility to 17 antimicrobials by means of a microbroth dilution test. Prevalence of antimicrobial resistance was compared between organic and conventional dairy farms.


RESULTS
E. coli was isolated from 1,121 (94%) fecal samples. Farm type (organic vs conventional) and animal age (cow vs calf) were significantly associated with odds that E. coli isolates would be resistant to various antimicrobials. After controlling for age, logistic regression analyses indicated that isolates from conventional dairy farms had significantly higher rates of resistance to ampicillin, streptomycin, kanamycin, gentamicin, chloramphenicol, tetracycline, and sulfamethoxazole than did isolates from organic dairy farms. However, no significant differences were detected for the 10 other antimicrobials that were tested.


CONCLUSIONS
Results indicated that compared with isolates from conventional dairy farms, E. coli isolates from organic dairy herds have significantly lower prevalences of resistance to 7 antimicrobials; however, prevalence of resistance was not significantly different for 10 other antimicrobials. Resistance was more common for isolates from calves than for isolates from adult dairy cows.
Sato
Kenji
K
Population Medicine Center, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824-1314, USA.
Bartlett
Paul C
PC
Saeed
Mahdi A
MA
D000367:Age Factors; D000822:Animal Husbandry / Q000379:methods*; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002417:Cattle; D002418:Cattle Diseases / Q000188:drug therapy*; D015169:Colony Count, Microbial / Q000662:veterinary; D003430:Cross-Sectional Studies; D024881:Drug Resistance, Bacterial; D004926:Escherichia coli / Q000187:drug effects*; D005243:Feces / Q000382:microbiology; D005260:Female; D008826:Microbial Sensitivity Tests / Q000662:veterinary; D014922:Wisconsin
D000900:Anti-Bacterial Agents