Sequential emergence of colistin and rifampicin resistance in an OXA-72- producing outbreak strain of Acinetobacter baumannii.

Here we report a hospital outbreak due to an extensively drug-resistant (XDR), OXA-72-producing strain of A. baumannii. The isolates were found to be genotypically indistinguishable by whole-genome MLST, and to belong to the international clonal complex CC2. One of these isolates sequentially developed a high resistance to colistin and rifampicin under treatment as a result of mutations in genes pmrB and rpoB, respectively. The blaOXA-72 gene was localized on a 10-kb transferable plasmid, named pAB-STR-1, whose sequence is nearly identical to that of another plasmid previously found in Lithuanian strains, pAB120. This report highlights the need to carefully monitor the emergence of colistin and rifampicin resistance in patients treated for infections with multidrug-resistant A. baumannii.