Molecular epidemiology of carbapenem-non-susceptible Acinetobacter baumannii in Japan

Acinetobacter baumannii presents a clinical challenge when it is non-susceptible to carbapenems. The prevalence of carbapenem-non-susceptible A. baumannii in Japan is unclear, as previous studies have been limited in scope. We investigated the spread of carbapenem-non-susceptible A. baumannii in Japan and performed a comparison with findings from overseas. A total of 305 non-duplicate clinical isolates of Acinetobacter spp. from 176 medical facilities in all geographical regions of Japan were tested for susceptibility to antimicrobial agents by the agar dilution method. Isolates with MICs of imipenem epsilon 4 mg/L underwent PCR analysis of OXA-type -lactamase gene clusters and metallo--lactamase genes. These isolates were further analysed by sequencing of OXA-type -lactamases and by multilocus sequence typing (MLST). Fifty-five of the 305 clinical isolates had MICs of imipenem epsilon 4 mg/L. The OXA-51-like carbapenemase gene was detected in 52 of these 55 isolates. Within the OXA-51-like gene cluster, OXA-66 was found in 43 (82.7) of the 52 isolates. MLST identified the following sequence types (STs): ST74, ST76, ST92, ST106, ST188 and ST195 in 2 (3.8), 2 (3.8), 40 (76.9), 5 (9.6), 2 (3.8) and 1 (1.9) of the isolates, respectively. In particular, ST92 was found in 31 (91.2) of the 34 A. baumannii isolates with MICs of imipenem epsilon 16 mg/L. This is the first report on the molecular epidemiology of A. baumannii with MICs of imipenem epsilon 4 mg/L in Japan. OXA-66 and ST92 were dominant among these isolates.