title: Prevalence and diversity of class 1 integrons and resistance genes in antimicrobial-resistant Escherichia coli originating from beef cattle administered subtherapeutic antimicrobials.
abstract: OBJECTIVE
  To characterize class 1 integrons and resistance genes in tetracycline-resistant Escherichia coli originating from beef cattle subtherapeutically administered chlortetracycline (A44), chlortetracycline and sulfamethazine (AS700), or no antimicrobials (control).
  
  
  RESULTS
  Tetracycline-resistant E. coli (control, n = 111; AS700, n = 53; A44, n = 40) were studied. Class 1 integrons, inserted gene cassettes and the presence of other antimicrobial resistance genes, as well as phylogenetic analysis, were performed by PCR, restriction enzyme analysis and sequencing. Susceptibilities to 11 antimicrobials were conducted on all isolates. Prevalence of class 1 integrase was higher (P < 0·001) in isolates from AS700 (33%) and A44 (28%) steers as compared to control (7%). Most integron gene cassettes belonged to the aad or dfr families. Correlations were found between the tet(A) gene and the genetic elements sul1 (r = 0·44), aadA1 (r = 0·61), cat (r = 0·58) and intI1(r = 0·37). Both closely and distantly related isolates harboured integrons with identical gene cassette arrays.
  
  
  CONCLUSIONS
  Subtherapeutic administration of chlorotetracycline alone or in combination with sulfamethazine may select for class 1 integrons in bovine tetracycline-resistant E. coli isolates. Vertical spread and horizontal transfer are responsible for the dissemination of a particular type of class 1 integron, but this study could not differentiate if this phenomenon occurred within or outside of the feedlot. Tetracycline-resistant E. coli strains with sul1 and tet(A) genes were more likely to harbour class 1 integrons.
  
  
  CONCLUSIONS
  Subtherapeutic use of chlortetracycline and sulfamethazine may promote the presence of class 1 integrons in tetracycline-resistant E. coli isolated from feedlot cattle.
authors.affiliation: College of Animal Science, Inner Mongolia Agricultural University, Hohhot, Inner Mongolia, China.
authors.forename: R B
authors.initials: RB
authors.lastname: Wu
authors.forename: T W
authors.initials: TW
authors.lastname: Alexander
authors.forename: J Q
authors.initials: JQ
authors.lastname: Li
authors.forename: K
authors.initials: K
authors.lastname: Munns
authors.forename: R
authors.initials: R
authors.lastname: Sharma
authors.forename: T A
authors.initials: TA
authors.lastname: McAllister
mesh_terms: D000818:Animals; D000900:Anti-Bacterial Agents / Q000008:administration & dosage* / Q000494:pharmacology; D002417:Cattle / Q000382:microbiology*; D002751:Chlortetracycline / Q000008:administration & dosage / Q000494:pharmacology; D004269:DNA, Bacterial / Q000235:genetics; D004926:Escherichia coli / Q000145:classification / Q000235:genetics / Q000302:isolation & purification*; D004927:Escherichia coli Infections / Q000382:microbiology / Q000662:veterinary; D029968:Escherichia coli Proteins / Q000235:genetics; D019426:Integrases / Q000235:genetics; D032023:Integrons; D008297:Male; D010802:Phylogeny; D016133:Polymerase Chain Reaction; D013418:Sulfamethazine / Q000008:administration & dosage / Q000494:pharmacology; D013753:Tetracycline Resistance / Q000235:genetics*
chemical_list: D000900:Anti-Bacterial Agents; D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; D013418:Sulfamethazine; D019426:Integrases; D002751:Chlortetracycline