OXA-23 and OXA-40 producing carbapenem-resistant Acinetobacter baumannii in Central Illinois

We reviewed susceptibility of 840 A. baumannii complex isolates at two academic medical centers and explored their mechanism of carbapenem resistance. Carbapenem resistance rates among A. baumannii increased from <5% before 2005 to 55% in 2011 and declined thereafter. We subjected 86 isolates for further antibiotic susceptibility testing using E-test, screened for MBL and carbapenemase production, and performed PCR for bla_OXA genes. Statistical analyses included correlation of resistance genes with susceptibility. Sixty-one isolates were non-susceptible to carbapenems (MIC >2 μg/mL). Phenotypic screening showed carbapenemase production in 50 isolates, but none was positive for MBL. Among carbapenem non-susceptible isolates, the CHDL (group D carbapenemase) encoding genes bla_OXA-23 (52%) and bla_OXA-40 (28%) were the most frequent genes. In conclusion, carbapenem resistance rates in A. baumannii peaked in 2011 and have since declined in our region. Carbapenem resistance among A. baumannii was primarily associated with production of acquired CHDLs including OXA-23 and OXA-40.