Prevalence and characteristics of aac(6′)-Ib-cr in AmpC-producing Enterobacter cloacae, Citrobacter freundii, and Serratia marcescens: a multicenter study from Korea

We investigated the prevalence of aac(6′)-Ib-cr and its association with other resistance genes in AmpC-producing Enterobacteriaceae without any selection criteria. A total of 479 clinical isolates of Enterobacter cloacae (179), Citrobacter freundii (134), and Serratia marcescens (166) from 12 laboratories between March and July 2005 were examined. We performed polymerase chain reaction for aac(6′)-Ib, blaOXA-1, ISEcp1, and class 1 integron. The aac(6′)-Ib-cr was further identified by digestion with BstF5I and sequencing. The aac(6′)-Ib was detected in 110 (23%) of 479 isolates, and 15 isolates (3.1%) were cr variants (8 E. cloacae, 5 C. freundii, and 2 S. marcescens). The aac(6′)-Ib-cr was significantly associated with various resistance genes (blaOXA-1, qnrS, qnrA, blaCTX-M-3, and blaCTX-M-14), mobile elements (ISEcp1, ISCR1, and class 1 integron), and quinolone resistance. Eleven of 15 aac(6′)-Ib-cr producers coharbored qnr genes. Although aac(6′)-Ib-cr was uncommon in Korean AmpC producers, its association with various resistance genes and mobile elements would facilitate the dissemination of this variant.