Tn3-associated rmtB together with qnrS1, aac(6′)-Ib-cr and blaCTX-M-15 are co-located on an F49:A-:B- plasmid in an Escherichia coli ST10 strain in China

Sir, Up to now, seven kinds of 16S rRNA methylase genes (armA, rmtA, rmtB, rmtC, rmtD, rmtE and npmA), whose products confer high-level resistance to aminoglycosides, have been reported in various Enterobacteriaceae strains.1 More recently, the co-prevalence of these genes with plasmid-mediated quinolone resistance determinants [qnrA, qnrB, qnrS, qnrC, qnrD, aac(6′)-Ib-cr and qepA] or extended-spectrum β-lactamases (ESBLs) has also been described in human and veterinary medicine.2,3 This tendency has greatly compromised the clinically important aminoglycosides, fluoroquinolones and β-lactams, presenting a major challenge for physicians when dealing with severe infectious diseases, due to the more limited options. However, studies focusing on whether these multiple resistance combinations co-exist on one plasmid remain scarce. Here, we identified that Tn3-associated rmtB together with qnrS1, aac(6′)-Ib-cr and blaCTX-M-15 are co-located on an F49:A-:B- plasmid in an Escherichia coli sequence type (ST) 10 strain isolated from bovine mastitis cases in China. In a survey of antimicrobial-resistant strains isolated from bovine mastitis cases in 2009, seven putative E. coli strains were initially isolated from 16 milk samples of bovine mastitis cases using a selective medium (CHROMagar™, France) in Henan Province, China. The MICs of five common antimicrobial agents (amikacin, ciprofloxacin, florfenicol, tetracycline and ceftiofur) for these strains were determined via the broth microdilution method on Mueller–Hinton medium according to guidelines issued by the CLSI. E. coli ATCC 25922 was used as a control. Of them, strain B09-2 exhibited high-level resistance to amikacin (MIC >1024 mg/L). This strain was further identified as an E. coli strain using a Vitek Gram-Negative Plus identification card (bioMérieux, France) and was subsequently designated as EB-01. The MIC results revealed that strain EB-01 was also resistant to ciprofloxacin and ceftiofur (MICs >128 and >128 mg/L, respectively). Multiplex PCR was performed to determine whether the strain EB-01 was commensal (A and B1) or was associated with phylogroups exhibiting extraintestinal virulence (B2 and D).4 The results revealed that this strain belonged to phylogroup A. Strain EB-01 was also analysed by multilocus sequence typing (‘MLST’) according to the protocols recommended at http://mlst.ucc.ie/mlst/dbs/Ecoli and assigned ST10.