Mutations conferring colistin resistance development in Acinetobacter baumannii clinical isolates
The colistin resistance mechanisms in Acinetobacter baumannii are largely unknown. In the present study, we investigated the characteristics and the mechanisms of colistin resistance in two genetically indistinguishable with each other colistin-susceptible/colistin-resistant (ColS/ColR) pairs of A. baumannii clinical isolates. The study included multiple ColS/ColR A. baumannii clinical isolates successively recovered from various clinical specimens of two patients during their hospitalization in ICU. Between ColS/ColR isolations the patients received colistin for several days. Susceptibility status was determined by Etest and broth dilution MIC testing and then the isolates tested by PFGE typing. The first ColS and the first ColR isolate from each patient were further investigated by MLST typing, PCR and sequencing of genes pmrA, pmrB, pmrC, lpxA, lpxC and lpxD and quantitative real-time PCR for testing the expression of the pmrA, pmrB, and pmrC genes. Finally, growth curves were performed in the two pairs of clinical isolates and the colistin-susceptible ATCC19606 control strain. ColS/ColR strains of each pair were genetically indistinguishable by PFGE and were assigned by MLST to the worldwide predominant international clone 2. Compared with the ColS, the ColR isolates overexpressed significantly the pmrCAB genes, each had single aminoacid shifts in PmrB protein, and exhibited significantly slower growth. ColS isolates of the first pair caused sequential bloodstream infections and ColS isolates of the second pair a severe soft tissue infection, while the respective ColR isolates were mainly colonizers. These findings overall indicate that prolonged colistin treatment contributed to the selection of ColR A. baumannii clinical isolates, which exhibited considerably lower infectivity than the susceptible ones.
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