In vivo selection of carbapenem resistance in Acinetobacter baumannii mediated by ISAba1 mobilisation upstream of the blaOXA-51 natural oxacillinase gene

Abstract number: P2012

Figueiredo S., Poirel L., Croize J., Recule C., Nordmann P.

Objectives: To analyse the mechanism(s) involved in the acquired resistance to carbapenems in A. baumannii under in vivo imipenem selection pressure.

Methods: Species identification was done by the biochemical API32 GN test (bio-Mérieux, France) and by 16S rRNA sequencing. Susceptibility testing was done by disk diffusion method and Minimum Inhibitory Concentration (MICs) were determined by agar dilution. Extended-spectrum b-lactamase (ESBL) was revealed by performing double-disk synergy test using disks containing ceftazidime or cefepime and ticarcillin-clavulanic acid on Mueller-Hinton agar plates. AmpC cephalosporinase hyperproduction was evaluated by performing ceftazidime susceptibility testing on cloxacillin-containing plates. Genes coding for the carbapenem-hydrolysing class D b-lactamases OXA-23, OXA-40, OXA-58 and OXA-51 subgroups were searched along with the presence of blaTEM-type and blaSHV-type genes. The isolates were analysed for the presence of ISAba1 in the promoter region of the blaOXA-51-like and blaampC genes in order to investigate the role of ISAba1 in blaOXA-51 and blaampC expression. Genotyping was done by pulsed field gel electrophoresis (PFGE) after digestion by ApaI.

Results: An imipenem-susceptible A. baumannii (A1) (MIC of 2 mg/L) isolate was recovered from rectal and wound swabs from an ICU patient. After ten days of imipenem treatment, an imipenem-resistant A. baumannii isolate (A2) (MIC 8 mg/L) was recovered from an endotracheal aspirate performed in the same patient. Genotyping revealed that both isolates were clonally-related. Both strains harboured blaTEM-1 gene and ISAba1 upstream of the blaampC gene leading to ticarcillin and ceftazidime resistance. Both strains had a naturally-occurring blaOXA-51-like gene with ISAba1 being identified upstream of the blaOXA-51 gene of A. baumannii A2, thus leading to overexpression of this carbapenem-hydrolysing oxacillinase.

Conclusion: This is the first report of an in vivo selection of carbapenem-resistance in A. baumannii. This acquired resistance trait was related to selection of an ISAba1 mobilisation event leading to blaOXA-51 overexpression.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Location: Barcelona, Spain
Presentation type:
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