Growth Retardation,Reduced Invasiveness,and Impaired Colistin-Mediated Cell Death Associated with Colistin Resistance Development in Acinetobacter baumannii

Two colistin-susceptible/colistin-resistant (Col^s/Col^r) pairs of Acinetobacter baumannii strains assigned to international clone 2, which is prevalent worldwide, were sequentially recovered from two patients after prolonged colistin administration. Compared with the respective Col^s isolates (Ab248 and Ab299, both having a colistin MIC of 0.5 μg/ml), both Col^r isolates (Ab249 and Ab347, with colistin MICs of 128 and 32 μg/ml, respectively) significantly overexpressed pmrCAB genes, had single-amino-acid shifts in the PmrB protein, and exhibited significantly slower growth. The Col^r isolate Ab347, tested by proteomic analysis in comparison with its Col^s counterpart Ab299, underexpressed the proteins CsuA/B and C from the csu operon (which is necessary for biofilm formation). This isolate also underexpressed aconitase B and different enzymes involved in the oxidative stress response (KatE catalase, superoxide dismutase, and alkyl hydroperoxide reductase), suggesting a reduced response to reactive oxygen species (ROS) and, consequently, impaired colistin-mediated cell death through hydroxyl radical production. Col^s isolates that were indistinguishable by macrorestriction analysis from Ab299 caused six sequential bloodstream infections, and isolates indistinguishable from Ab248 caused severe soft tissue infection, while Col^r isolates indistinguishable from Ab347 and Ab249 were mainly colonizers. In particular, a Col^s isolate identical to Ab299 was still invading the bloodstream 90 days after the colonization of this patient by Col^r isolates. These observations indicate considerably lower invasiveness of A. baumannii clinical isolates following the development of colistin resistance.