The Investigation Of The Carbapenem Resistance Mechanisms And Molecular Epidemiology Of Acinetobacter Baumannii

Objective:77consecutive, non-duplicate A.baumannii isolatesleading to the nosocomial outbreaks from September2007to September2009in our hospital were investigated for the carbapenem-resistantmechanisms, focusing on the influences of the class D β-lactamases(CHDLs) and the upstream insert sequence (IS) elements on carbapenemresistance of A. baumannii. Meanwhile, the molecular epidemiologyconcerning these isolates was explored.Methods: To differentiate A. baumannii from A. baumannii complex(ABC), PCR assays detecting blaOXA-51-likegenes were performed. Theantibiotic susceptibility tests were performed using the Vitek2compactsystem with the AST-GN09cards (bioMerieux, Hazelwood, MO, France).PCR assays were used for detecting encoding genes of MBLs (includingblaVIM, blaIMP, blaSIMand blaNDM-1) and those of CHDLs (includingblaOXA-23-like, blaOXA-24-like, blaOXA-51-likeand blaOXA-58-like) and the insertsequence (IS) elements. Pulsed Field Gel Electrophoresis (PFGE), MultipleLocus Sequence Typing (MLST) and identification of European clones were implemented for the epidemiological investigation.Results:1. According to antibiotic susceptibility tests, isolates weredevided into two groups, i.e., the carbapenem-resistant A. baumannii(CRAB) group (57isolates) and non-carbapenem-resistant A. baumannii(non-CRAB) group (20isolates).2. Negative results were acquired for MBLs genes in all isolates,while all the57CRAB isolates were positive for blaOXA-23-likegenes. Inthese57isolates, ISAba1showed positive result when amplified withspecific primers for ISAba1-blaOXA-23-likegenes. In the20non-carbapenem-resistant A. baumannii isolates, two gave positive resultsfor both ISAba1-blaOXA-51-likeand blaOXA-58-likegenes. blaOXA-24-likegeneswere negative in all the77isolates.3. PFGE results showed that77isolates belonged to six clone types(A to F), with the A and B clone types being the dominant PFGE clonetypes leading to the outbreaks. The A and B clone types each contained30and37isolates, respectively. There were4sub-clone types in A clonewhile B got5.4. Representatives of each PFGE clone type were picked up forMLST, and ST75(A clone), ST137(B clone) and ST17(E clone) wereidentified. Besides these, ST N1（C and F clone）and ST N2（D clone）were firstly discovered.5. Identification of European clones demonstrated that the ST75(A clone) and ST N1（C and F clone）both belonged to European clone II.Conclusions: The majority of the77isolates werecarbapenem-resistant A. baumannii (57isolates,74%), clone A (ST75,European clone II) and clone B (ST137) CRAB isolates that produced theblaOXA-23-likegene with an upstream ISAba1contributed to the nosocomialoutbreaks studied in this work.