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Title: Effects of tylosin use on erythromycin resistance in enterococci isolated from swine

Author
item Jackson, Charlene
item Cray, Paula
item Barrett, John
item Ladely, Scott

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2004
Publication Date: 7/20/2004
Citation: Jackson, C.R., Cray, P.J., Barrett, J.B., Ladely, S.R. 2004. Effects of tylosin use on erythromycin resistance in enterococci isolated from swine. Applied and Environmental Microbiology. 70(7):4205-4210.

Interpretive Summary: Evaluation of the effects of antimicrobial use on bacterial resistance is an important process. In this study, the effect of tylosin use on erythromycin (erm) resistance in enterococci from swine was determined. Three farms were evaluated: farm A where tylosin was used for growth-promotion, farm B where tylosin was used to treat pigs, and farm C where no tylosin was used. Higher levels of erm resistant enterococci were isolated from farm A (59%) followed by farm B (28%) and farm C (2%). Resistance was due to the presence of ermB located primarily on the chromosome of the isolates, but also on plasmids in a minority of the isolates. The selective pressure for maintenance of erm resistance on farm C remains to be determined. This information will be useful for researchers who study the effects of antimicrobial use in animals and the potential effects on humans.

Technical Abstract: The effect of tylosin on erythromycin resistant enterococci was examined on three farms, farm A that used tylosin for growth-promotion, farm B that used tylosin for treatment of disease, and farm C where no tylosin was used. Six hundred and sixty-two enterococci were isolated from gestation, farrowing, suckling, nursery, and finishing swine from these farms. Of these, 59%, 28%, and 2% were resistant to erythromycin (MIC greater than or equal to 8 ug/ml) from farm A, B, and C, respectively. PCR analysis and Southern blotting revealed that 65 isolates chosen from all three farms for further study were positive for ermB, but negative for ermA and ermC. Using Southern blotting, ermB was localized to the chromosome in 56 of the isolates while 9 isolates from farm A and B contained ermB on two similar sized plasmids (12-16 kb). Pulsed-field gel electrophoresis revealed that the isolates were genetically diverse and represented a heterogeneous population of enterococci. This study suggested that although resistance was higher on a farm where tylosin was used as a growth promotant, resistant enterococci also persisted on a farm where no antimicrobials were used.