Structure analysis of transposons carrying the aac(6′)-aph(2″) gene in Enterococcus faecalis isolated in Beijing, China, and comparison of their transfer efficiency

•Two new aac(6′)-aph(2″) transposon structures were found in Enterococcus faecalis from Beijing, China.•Truncated aac(6′)-aph(2″) transposons were not always related to high plasmid location rate and high transfer efficiency.•Truncated aph(2″)-Ia region was identified upstream of a type III transposon with different rates among transposon groups.•Truncated aph(2″)-Ia region had similar positive rate trend as transposon plasmid location and conjugation-positive rates. Transfer of aac(6′)-aph(2″) transposons mediating high-level gentamicin resistance (HLGR) in Enterococcus faecalis is a serious problem in the clinic. However, factors affecting the transfer of aac(6′)-aph(2″) have not yet been elucidated. The current study aimed to examine the genetic and molecular basis of HLGR in E. faecalis strains isolated in Beijing (China) and to clarify the relationship between transfer efficiency of aac(6′)-aph(2″) transposons and the transposon structure/location. A total of five transposon structures were identified by PCR mapping of the corresponding transposon regions, including a Tn5281-like non-truncated transposon and four truncated transposons. A plasmid location study of aac(6′)-aph(2″) by Southern blot following S1-PFGE and filter mating conjugation experiments demonstrated that plasmid location rates correlated with conjugation-positive rates. Chromosome walking to identify the sequence upstream of a representative type III truncated transposon found a truncated aph(2″)-Ia region, and further PCR analysis of this region among strains from different groups revealed similar a positive rate trend as the transposon plasmid location rate and conjugation-positive rate. In conclusion, aac(6′)-aph(2″) transposons were of different structures in E. faecalis strains from Beijing, with two new transposon structures that have not been reported elsewhere. Presence of the truncated aph(2″)-Ia region upstream of some truncated transposons suggests recombination between aminoglycoside-modifying enzyme genes. Possible links exist among plasmid location, conjugation and the presence of truncated aph(2″)-Ia upstream of the transposon.