Spread of a carbapenem- and colistin-resistant Acinetobacter baumannii ST2 clonal strain causing outbreaks in two Sicilian hospitals

Background: Infections caused by multidrug-resistant (MDR) Acinetobacter baumannii have become an important healthcare-associated problem, particularly in intensive care units (ICUs). Aim: To investigate the emergence of carbapenem- and colistin-resistant A.baumannii infections in two Sicilian hospitals. Methods: From October 2008 to May 2011, a period which included two Italian Nosocomial Infections Surveillance in ICUs network (SPIN-UTI) project surveys, all carbapenem-resistant A.baumannii isolates from the ICUs of two hospitals in Catania, Italy, were prospectively collected. Minimum inhibitory concentrations (MICs) were measured by agar dilution, and phenotypic testing for metallo-β-lactamase (MBL) production was performed. Carbapenem resistance genes and their genetic elements were identified by polymerase chain reaction and sequencing. Genotypic relatedness was assessed by pulsed-field gel electrophoresis (PFGE) and multi-locus sequence typing. Patient-based surveillance was conducted using the SPIN-UTI protocol and previous antibiotic consumption was recorded. Findings: Twenty-six carbapenem-resistant A.baumannii were identified. Imipenem and meropenem MICs ranged from 4 to >32mg/L, and 15 isolates exhibited high-level colistin resistance (MICs >32mg/L). PFGE demonstrated that all isolates belonged to a unique clonal type and were assigned to ST2 of the international clone II. They harboured an intrinsic bla_OxA-51-like carbapenemase gene, bla_OxA-82, which was flanked upstream by ISAba1. Conclusions: The dissemination of clonally related isolates of carbapenem-resistant A.baumannii in two hospitals is described. Simultaneous resistance to colistin in more than half of the isolates is a problem for effective antibiotic treatment. Prior carbapenem and colistin consumption may have acted as triggering factors.