The genetic environment of armA on pHNE, an IncN plasmid, in one Escherichia coli isolate from a chicken

Sir, Methylation of 16S ribosomal RNA (rRNA) mediated by 16S rRNA methylase, which confers high-level aminoglycoside resistance (MICs .1024 mg/L) in Enterobacteriaceae, has recently emerged as a major medical problem worldwide. Until now, seven plasmid-encoded 16S rRNA methylases, ArmA, RmtA, RmtB, RmtC, RmtD, RmtE and NpmA, have been reported in various clinical Gram-negative isolates in multiple geographic regions. armA is one of the most prevalent 16S rRNA methylase genes found in several Gram-negative pathogens. Since it was initially identified in Klebsiella pneumoniae in 2003, two studies focusing on its genetic environment and dissemination in Enterobacteriaceae from both humans and animals have been conducted. Here, we identify a novel genetic environment of armA on pHNE, an IncN plasmid, in one Escherichia coli isolate from a chicken. Previously, we reported the emergence of 16S rRNA methylases encoded by armA and rmtB in E. coli isolates from chickens. In a routine survey of antimicrobial-resistant isolates from chickens in 2009, armA was detected in an E. coli isolate exhibiting high-level resistance to the aminoglycoside antibiotic amikacin (MICs .1024 mg/L) (n1⁄41/18) from a diseased flock in a typical traditional farm in Henan Province, China. Phylogenetic group analysis using multiplex PCR revealed that this armA E. coli isolate belonged to phylogenetic group D, which was associated with extra-intestinal virulence. In order to further elucidate the genetic environment of armA, plasmids extracted from this armA-positive isolate (using the Plasmid Midi Kit, Qiagen, Germany) were transferred to E. coli DH10B by electroporation to investigate whether the armA resistance determinant was localized on plasmids. The transformants were screened on a Luria–Bertani agar plate containing