Wide spread of Tn2006 in an AbaR4-type resistance island among carbapenem-resistant Acinetobacter baumannii clinical isolates in Taiwan

Carbapenem resistance in Acinetobacter baumannii is a global problem. The purpose of this study was to elucidate current resistance mechanisms of imipenem-resistant A. baumannii (IRAB) in Taiwan and their correlation with patient outcomes. Acinetobacter baumannii clinical isolates from two teaching hospitals in Taiwan were collected in 2009 and were examined by Etest for determination of the minimum inhibitory concentrations (MICs) of imipenem, ceftazidime and ceftriaxone. Primers specific for carbapenemase genes and upstream regions were designed for PCR amplification. Bacterial isolates were genotyped by pulsed-field gel electrophoresis (PFGE). Clinical presentations of patients were analysed retrospectively. Upstream insertion sequence ISAba1 was found in 34 isolates that carried bla(OXA-23), including 28 with transposon Tn2006 (ISAba1-bla(OXA-23)-ISAba1) in an AbaR4-type resistance island and 6 with Tn2008 (ISAba1-bla(OXA-23)), as well as in 8 isolates carrying ISAba1-bla(OXA-51-like). All of these isolates expressed full resistance to imipenem (MIC>32 mg/L). Forty-one different PFGE genotypes were found among 62 isolates. Tn2006 was found in 19 genotypes (46.3%), which is more common than ISAba1-bla(OXA-51-like) (12.2%) (P=0.001). Prior use of carbapenems or extended-spectrum cephalosporins for ≥5 days was the only independent risk factor significantly associated with IRAB infection (odds ratio=361.175). Higher mortality was significantly associated with infection caused by IRAB and ISAba1-bla(OXA-23)-carrying strains compared with infection caused by imipenem-susceptible A. baumannii and ISAba1-bla(OXA-51-like)-carrying strains (P=0.009 and 0.027, respectively). Tn2006 is currently the most common imipenem resistance determinant, which showed a higher ability to spread among A. baumannii and was associated with a higher mortality in IRAB-infected patients.