| Summary: | Horizontal gene transfer events provide the basis for extensive dissemination of antimicrobial resistance traits between bacterial populations. Conjugation is considered to be the most frequent mechanism behind new resistance acquisitions in clinical pathogens but does not fully explain the resistance patterns seen in some bacterial genera. Gene transfer by natural transformation has been described for numerous clinical isolates, including some <i>Acinetobacter</i> species. The main aim of this study was to determine to what extent clinical, resistant <i>Acinetobacter</i> spp. isolates, express competence for natural transformation. Twenty-two clinical <i>Acinetobacter</i> spp. isolates collected over a 16-year time period, from five different geographical separated and/or distinct Portuguese Hospitals were tested for natural transformability. Fourteen isolates, including 11 <i>A. baumannii</i>, 2 <i>A. nosocomialis</i> and 1 <i>Acinetobacter</i> sp., were identified as competent on semisolid media facilitating surface-motility. Competent <i>Acinetobacter</i> isolates were found in all the hospitals tested. Furthermore, osmolarity was shown to influence the uptake of exogenous DNA by competent <i>A. baumannii</i> A118. Our study demonstrates that natural competence is common among clinical isolates of <i>Acinetobacter</i> spp., and hence likely an important trait for resistance acquisition.
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