Evolution of multidrug-resistant Acinetobacter baumannii isolates obtained from elderly patients with respiratory tract infections

Objectives: To study the evolution between 1999 and 2002 and mechanisms of antibiotic resistance in a multidrug-resistant Acinetobacter baumannii clone predominant in isolates from elderly patients with respiratory tract infections.Methods: Susceptibility to antimicrobials was determined using an agar dilution method. Bacterial clones were identified by PCR-fingerprinting and PFGE with ApaI. Carbapenemases were detected by phenotypic tests; by PCR with primers specific for blaOXA-40, blaIMP, blaVIM-1 and blaVIM-2; and by hybridization with DNA probes. Class 1 integrons were detected using PCR.Results: In 1999 isolates were grouped into two main genotypes: clone I (33%) and clone II (55%). These were also detected in 2002 with a different distribution: clone I (69%), clone II (22%). Resistance to amikacin, meropenem and imipenem increased significantly in clone I over this time, whereas clone II was not affected. In 2002, the incidence of blaOXA-40 rose to 91% in clone I isolates with some also harbouring blaVIM-2 and blaIMP genes. Different class 1 integrons were detected ranging in size from 550 to 1200 bp. No relationship was found between carbapenemases and class 1 integrons.Conclusions: In elderly patients, a single clone became predominant among A. baumannii isolates, coinciding with an increase in antibiotic resistance rates. The majority of isolates harboured the blaOXA-40 carbapenemase gene and some of them also harboured blaVIM-2 and blaIMP genes. The presence of class 1 integrons also increased over time.