The Evolution of Multidrug Resistance in a Hospital Pathogen

The recent emergence of multidrug resistance(MDR) in Acinetobacter baumannii has raised concern in health care settings worldwide. In order to understand the repertoire of resistance determinants and their organization and origins, we compared the genome sequences of three MDR and three drug-susceptible A. baumannii isolates. The entire MDR phenotype can be explained by the acquisition of discrete resistance determinants distributed throughout the genome. A resistance island (RI) with a variable composition of resistance determinants interspersed with transposons, integrons, and other mobile genetic elements is a significant, but not universal, contributor to the MDR phenotype. Variable resistance gene composition amongidentical clone types from a single outbreak suggests dynamic and active horizontal transfer. 475 genes are shared among all six clinical isolates, but absent from the related environmental species Acinetobacter baylyi ADP1.These genes are enriched for transcription factors and transporters and suggest physiological features of A.baumannii that are related to adaptation for growth in association with humans.