Distribution and genetic composition of carbapenem-resistance determinants in clinical <em>Acinetobacter</em> isolates – focus on carbapenem-hydrolysing β-lactamases

Abstract

<i>Acinetobacter baumannii</i> is of global concern in health-care facilities as it can easily thrive in the hospital environment and infections can be difficult to treat. Widespread multi-drug (MDR) resistance and a potential for epidemic spread contribute to the worldwide success of this hospital-acquired pathogen. Currently the drugs of choice to treat infections with MDR <i>A. baumannii</i> are carbapenems, but resistance is rising. Commonly carbapenem resistance in <i>A. baumannii</i> is based on enzymatic inactivation of the agent, for example by metallo-ß-lactamases like NDM-1 and by carbapenem-hydrolysing oxacillinases (OXA), including the intrinsic OXA-51 subclass and the acquired subclasses OXA-23, OXA-40, OXA-58 and OXA-143. Even though OXA weakly hydrolyse carbapenems they are often associated with insertion sequences (IS) which can provide strong promoters and lead to OXA overexpression. <br /> The aim of this work was to investigate carbapenem-resistance mechanisms in <i>A. baumannii</i> and non-<i>baumannii Acinetobacter</i> spp. For the first time the genetic context of a chromosomally encoded _NDM-1 in <i>A. baumannii</i> was analysed. The gene was located on a transposon flanked by IS<i>Aba125</i> that had inserted into a chromosomal transporter gene. Furthermore eighteen novel OXA genes, one OXA subclass (OXA-235-like) and one insertion sequence (IS<i>Acra1</i>) were identified. It was shown that detection of the intrinsic <i>bla</i>_OXA-51-like by multiplex PCR is not reliable as the sole method for <i>A. baumannii</i> species identification. OXA-51-like sequencing and rep-PCR-based DiversiLab revealed correlation of <i>bla</i>_OXA-51-like variants with <i>A. baumannii</i> international clonal lineages IC1 to IC8. In outbreak-related <i>A. baumannii</i> isolates a conversion of the intrinsic OXA from OXA-66 to OXA-82 was found to be associated with carbapenem resistance and presence of IS<i>Aba1</i>. Investigation of closely related OXA-66, OXA-82 (L167V), OXA-109 (P130Q) and OXA-201 (L167V and P130Q) on an isogenic background indicated that carbapenem resistance based on IS<i>Aba1</i>-mediated overexpression of the intrinsic OXA is dependent on the OXA-51 variant.<br /> For the first time carbapenem resistance based on IS-mediated overexpression of an intrinsic OXA was observed in <i>A. radioresistens</i> and <i>A. bereziniae</i>. Furthermore OXA-23 was initially detected in <i>Acinetobacter genomic species 14TU/13BJ and OXA-143, as well as co-expressed OXA-23 and OXA-58 were identified in <i>A. pittii</i> isolates. Emergence of OXA-143-like in <i>A. baumannii</i> and <i>A. pittii</i> outside of Brazil and South Korea was observed. Investigation of carbapenem susceptibility under different physiological conditions in <i>A. baumannii</i> revealed a strain-dependent effect of NaCl and sodium-salicylate, while pH 5.8 reduced carbapenem susceptibility of all OXA-58-like-expressing transformants from a resistant (=32 mg/L) to a non-susceptible level (8 mg/L).<br /> Taken together this work illustrates the diversity and variability of carbapenem-hydrolysing ß-lactamases in <i>Acinetobacter</i> spp. and leads to the hypothesis that carbapenem resistance will further disseminate in the <i>Acinetobacter</i> genus in the foreseeable future. </i>