Emergence and spread of a multidrug-resistant Acinetobacter baumannii clone producing both the carbapenemase OXA-23 and the 16S rRNA methylase ArmA

Acinetobacter baumannii is a Gram-negative organism reported worldwide as a cause of health-care-associated infections, particularly in intensive care units (ICUs). The aim of this study is to describe the emergence and spread of carbapenem-resistant A. baumannii (CRAB) isolates in hospitalized patients. From March to November 2009, multidrug-resistant CRAB isolates were obtained from 21 patients hospitalized in different wards (mostly ICUs). Antimicrobial susceptibility was determined by using the Etest method. Carbapenem and aminoglycoside resistance determinants were studied by PCR and sequencing. Genetic relatedness was investigated by pulsed-field gel electrophoresis and multiplex PCR identification of sequence groups. Clinical records of patients were examined retrospectively. CRAB isolates were consistently resistant to multiple drugs including fluoroquinolones and aminoglycosides, whereas they retained a susceptibility to colistin. Molecular analysis revealed that 19 of the 21 CRAB isolates belonged to a single clone producing both the carbapenemase OXA-23 and the 16S rRNA methylase ArmA. Based on clinical data, the patients included in the study were classified as infected (n=13) or colonized (n=8). Colistin alone or in combination with ampicillin-sulbactam was administered to 11 of the 13 infected patients. A complete or partial response was obtained in eight cases, whereas a failure to respond was observed in one patient and a relapse was observed in two patients. An A. baumannii clone producing both OXA-23 and ArmA has been identified as an emerging and rapidly spreading pathogen. To our knowledge, this is the first report of the ArmA enzyme in A. baumannii in Italy and is the first report of hospital dissemination of A. baumannii carrying both bia(OXA-23) and armA genes.