JAC Advance Access published online on March 12, 2007
Journal of Antimicrobial Chemotherapy, doi:10.1093/jac/dkm065
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Emergence of RmtB methylase-producing Escherichia coli and Enterobacter cloacae isolates from pigs in China
College of Veterinary Medicine, Guangdong Provincial Key Laboratory of Veterinary Pharmaceutics Development and Safety Evaluation, South China Agricultural University, Guangzhou 510642, People's Republic of China
Received 27 October 2006; returned 12 November 2006; revised 7 February 2007; accepted 10 February 2007
* Corresponding author. Tel: +86-20-85280237; Fax: +86-20-85284896; E-mail: jhliu{at}scau.edu.cn
Objectives: To investigate the occurrence of 16S rRNA methylases conferring high-level resistance to aminoglycosides in Enterobacteriaceae isolated from two pig farms in China.
Methods: Enterobacteriaceae isolated from 151 pig rectal swab samples and 9 environmental samples were screened for the presence of the rmtA, rmtB, armA and rmtC genes by PCR and sequencing. Conjugation experiments were carried out to study the transferability of the 16S rRNA methylase genes. All isolates and their transconjugants were tested for susceptibility to antimicrobial agents. The clonal relatedness of RmtB-producing Escherichia coli was assessed by PFGE with XbaI.
Results: Of 152 Enterobacteriaceae isolates recovered from pigs, 49 (32%) were positive for the rmtB gene, including 48 E. coli and a single isolate of Enterobacter cloacae. Of the nine Enterobacteriaceae isolates from environmental samples, no 16S rRNA methylase gene was identified. The 49 rmtB-positive isolates showed resistance to ampicillin, tetracycline and trimethoprim and also carried a blaTEM gene. Transfer of the rmtB and blaTEM genes by conjugation experiments of all 49 isolates was successful, suggesting that the rmtB-containing plasmids in the E. coli and E. cloacae isolates were self-transmissible. Conjugative transfer frequencies varied from 2.2 x 10–10 to 1.3 x 10–6 transconjugants per recipient. The transfer of non-aminoglycoside antimicrobial resistance traits was also observed in most cases. Forty-four rmtB-positive E. coli showed 30 different PFGE types.
Conclusions: The rmtB gene was detected on conjugative plasmids of porcine E. coli and E. cloacae isolates. Both horizontal gene transfer and clonal spread were responsible for the dissemination of the rmtB gene. The emergence of 16S rRNA methylases in Enterobacteriaceae isolates is described for the first time in China. This is also the first report of rmtB-positive Enterobacteriaceae among healthy food-producing animals.
Key Words: 16S rRNA methylases , animal isolates , aminoglycoside resistance
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