engleski

Clonal spread of carbapenem non susceptible Acinetobacter baumannii

Background and aim: Sixty nine A. baumannii isolates were collected in 2009. from various clinical specimens and hospital units in the hospital in Pula, Croatia. The isolates were identified by conventional biochemical testing. Material and methods: The antimicrobial susceptibility to a wide range of antibiotics was determined by broth microdilution in Mueller-Hinton broth and 96 well microtitar plates according to CLSI guidelines. Minimum inhibitory concentrations (MICS) were determined for imipenem and meropenem by agar dilution in the presence of sodium chloride (200 mM) and cloxacillin (200 mg/L) in order to determine the effect of chromosomal AmpC β- lactamase on the susceptibility to carbapenems. E-test MBL strips were used for detection of metallo-carbapenemases following the manufacturer's instructions. A decrease of imipenem MIC by ≥3 twofold dilutions in the presence of EDTA was interpreted as being suggestive of MBL production. PCR was used to detect the presence of the genes encoding the MBLs of IMP and VIM series and, blaOXA (blaOXA- 51, blaOXA-23, blaOXA-40blaOXA-58and blaOXA- 143) genes as previously described. The genetic context of blaOXA-51 and blaOXA-58 genes was determined by PCR mapping with the primers for ISAbaI combined with forward and reverse primers for blaOXA-51 and with primers for ISAbaIII combined with forward and reverse primers for blaOXA-58. Sequence groups (1-3) corresponding to EU clones I-III were determined by multiplex PCR as described previously. PFGE genotyping of XbaI-digested genomic DNA was performed with a CHEF-DRIII system. The images were processed using Gel- Compar software, and a dendrogram was computed after band intensity correlation using global alignment with 2% optimization and UPGMA (unweighted pair-group method using arithmetical averages) clustering. The strains were considered to be clonally related if they showed more than 80% similarity of their PFGE patterns. Results: Above 90% of the strains were resistant to cefotaxime, ceftriaxone, piperacillin, piperacillin/tazobactam, ciprofloxacin and gentamicin. The strains showed variable degrees of susceptibility/resistance to imipenem and meropenem. 61% of the strains showed reduced susceptibility to meropenem and 59% to imipenem. Most strains were intermediate susceptible to both carbapenems with MICs around 8 mg/L. All strains were susceptible to colistin. Colistin was the most potent antibiotic with MIC90 of 2 mg/L. 12 isolates were found to produce OXA-24 and 21 to produce OXA58. Other isolates were positive only for naturally ocurring OXA-51 beta-lactamase which was associated with ISAbaI insertion sequence located upstream of blaOXA-51. No MBLs were found. Chromosomal AmpC β-lactamases did not affect the susceptibility to carbapenems. Sodium chloride did not decrease carbapenem MICs. The strains were found to belong to EU clone 1 (sequence group II). PFGE confirmed the clonality of the isolates. PFGE revealed several clonal types. Conclusions: This study demonstrated different types of oxacillinases in investigated strains. The presence of ISAbaI insertion sequence is thought to upregulate the expression of blaOXA-51 gene and therefore may be responsible for elevated carbapenem MICs. Colistin became the antibiotic of choice for the treatment of infections caused by our A. baumannii isolates as this was the only antimicrobial to have activity. However, the risk of nephrotoxicity is of clinical concern. In conclusion, this study highlights the need to establish an antimicrobial surveillance network for A. baumannii to monitor the trends and new types of resistance mechanisms. Furthermore, the factors responsible for dissemination of such strains need to be identified, controlled and prevented to avoid major outbreaks.

Acinetobacter baumannii; carbapenem

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