Emergence of aac(6')-Ie-aph(2'')-Ia-positive enterococci with non-high-level gentamicin resistance mediated by IS1216V: adaptation to decreased aminoglycoside usage in Taiwan.

OBJECTIVES

To explore the mechanisms mediating the different levels of gentamicin resistance in enterococci.

METHODS

Susceptibility testing with gentamicin and PCR of resistance determinants were performed in 149 enterococcal isolates. Genetic relatedness was characterized by MLST and PFGE analysis. Sequences of the aac(6')-Ie-aph(2'')-Ia gene and its surrounding environment were determined by Illumina sequencing. Stability assays of gentamicin resistance were carried out to evaluate the probability of loss of the high-level gentamicin resistance (HLGR) phenotype.

RESULTS

A total of 17 (11.4%) aac(6')-Ie-aph(2'')-Ia-positive enterococcal isolates (2 Enterococcus faecalis and 15 Enterococcus faecium) with non-HLGR phenotype were found. MLST analysis revealed that the 2 E. faecalis belonged to ST116 and ST618, while all the 15 E. faecium belonged to clonal complex 17. Sequence analysis demonstrated that IS1216V was inserted into the 5'-end of aac(6')-Ie-aph(2'')-Ia, leading to loss of HLGR phenotype. Three IS1216V insertion types were found, and type II and III were frequently found in E. faecium. Interestingly, a total of 38 aac(6')-Ie-aph(2'')-Ia-positive E. faecium with HLGR phenotype also had type II or type III IS1216V insertion. Sequencing of the aac(6')-Ie-aph(2'')-Ia-positive HLGR E. faecium E37 revealed that an intact aac(6')-Ie-aph(2'')-Ia was located adjacent to IS1216V-disrupted aac(6')-Ie-aph(2'')-Ia. In a non-antibiotic environment, E37 tended to lose HLGR phenotype with a probability of 1.57 × 10-4, which was largely attributed to homologous recombination between the intact and disrupted aac(6')-Ie-aph(2'')-Ia.

CONCLUSIONS

This is first study to elucidate that the E. faecium is capable of changing its HLGR phenotype, which may contribute to adaptation to hospital environments with decreased usage of gentamicin.