| 1. BackgroundAcinetobacter baumannii can be found in natural environment, hospital surroundings, and skin of human body. It is an important opportunistic nosocomial pathogen and particularly important in hospital acquired pneumonia, especially in immuno-compromised patients and those with tracheotomy or mechanical ventilation. A. baumannii isolates are usually multiresistant, and their infections are complicating therapy. Carbapenems have good antibacterial activity against A. baumannii. However, carbapenem-resistant isolates are increasingly isolated as carbapenems are used widely in clinical practice. Mechanisms of carbapenems resistance include: production of carbapenemase, loss of outer membrane porin, alteration of target, and efflux system. In recent years, the reports about acquired carbapenemase in frequently isolated pathogens such as P. areruginosa, A. baumannii, and Enterobacteriaceae has been increasing. The outbreak caused by these strains makes the situation worse. Few effective antimicrobial agents are available for these infections with high mortality.Carbapenemases may be defined as β -lactamases that significantly hydrolyze atleast imipenem or/and meropenem. They belong to Ambler molecular class A, class B and class D. The class B enzymes are metalloenzymes and hydrolyze virtually all β-lactams except aztronam, whose genes are chromosome, plasmid or integron located. The acquired metalloenzymes are plasmid or integron encoded in Psedomonas aeruginosa A. baumannii, and Enterobacteriaceae. Class A, clavulanic acid-inhibited carbapenemases, have only been reported in rare Enterobacterial isolates. They belong to the group 2f as defined by Bush et al. The class D carbepenemases belonging to the group 2d are from A. baumannii isolates, and hydrolyze imipenem and meropenem weakly.We carried out this study to demonstrate the resistance pattern and prevalence of carbapenems-resistant A. baumannii in the First hospital and Sir Run Run Shaw Hospital, affiliated to the college of medicine, Zhejiang University. We described the resistance pattern of 51 strains of imipenem-resistant A. baumannii to 14 antimicrobial agents and analyzed the homology between these strains and studied the carbapenemases produced.2. Materials and methodsThe homology of 51 strains of imipenem-resistant A. baumannii collected from clinical specimens was analyzed by pulsed-field gel electrophoresis (PFGE). The minimal inhibitory concentrations(MICs) of these strains were detected by Etest strips, and the carbapenemases produced by these strains were typed by isoelectric focusing (IEF) electrophoresis, three-dimensional test, 2-mercaptopropanoic acid inhibited assays.The encoding genes were analysed by polymerase chain reaction (PCR), cloning and sequencing.3. ResultsAll 51 strains were multi-drug resistant and also highly resistant to meropenem. Themost active agents against these strains were cefoperazone/sulbactam and ampicillin/sulbactam with susceptibility rate of 82% and 40%, respectively. About 66%~74% of these strains were intermediate to cefepime, ceftazidime and cefotaxime. All strains were highly resistant to other antimicrobial agents tested.Forty-six strains isolated from the First Hospital were classified into Type A, B and C based on PFGE pattern. Most of type A strains were isolated from ICU and were the dominant strains including subtypes A1, A2, A3 and A4. Only 1 strain isolated from hematology department belongs to Type B, and another strain isolated from ICU belongs to Type C. Forty-four strains produced carbapenemases. One strain ( PFGE type B) had 2 bands on IEF electrophoresis. The pIs of which were 6.7 and 7.2 respectively. The band with pI of 6.7 was OXA-23. Another strain(PFGE type C) produce a new carbepenemase(OXA-49,GenBank accession No.AY288523) being a point-mutant of OXA-23. The other 42 strains produced 2 β-lactamases bands with pIs of 6.4 and 7.0 that couldn't be inhibited by clavulanic acid, cloxacillin and 2-mercaptopropanoic acid, OXA-23, OXA-24 typ...
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