A novel gentamicin resistance gene in Enterococcus.

Enterococcus gallinarum SF9117 is a veterinary isolate for which the MIC of gentamicin is 256 micrograms/ml. Time-kill studies with a combination of ampicillin plus gentamicin failed to show synergism against SF9117. A probe representing aac(6')-aph(2") did not hybridize to DNA from SF9117. A 3.2-kb fragment from plasmid pYN134 of SF9117 was cloned and conferred resistance to gentamicin in Escherichia coli DH5 alpha. Nucleotide sequence analysis revealed the presence of a 918-bp open reading frame whose deduced amino acid sequence had a region with homology to the C-terminal domain of the bifunctional enzyme AAC(6')-APH(2"). The gene is designated aph(2")-Ic, and its observed phosphotransferase activity is provisionally designated APH(2")-Ic. An intragenic probe hybridized to the genomic DNA from an Enterococcus faecium isolate from the peritoneal fluid of one patient and to the plasmid DNA of an Enterococcus faecalis isolate from the blood of another patient. An enterococcal isolate containing this novel resistance gene might not be readily detected in clinical laboratories that use gentamicin at 500 or 2,000 micrograms/ml for screening for high-level resistance.