Abstract

The spread of extensively drug-resistant (XDR) gram-negative bacteria has boosted colistin use, with a resultant selection of colistin-resistant, often pandrug-resistant strains. Whether acquisition of further resistance mechanisms translates into a reduced virulence is the subject of active research. In this report, we describe clinical features of an immunocompromised patient who developed infection due to colistin-resistant Acinetobacter baumannii while on long-term colistin therapy. We analyzed phenotypic and genotypic characteristics, molecular mechanisms of colistin resistance, and in vitro and in vivo fitness of sequential colistin-sensitive and colistin-resistant strains isolated from the patient. Both colistin-sensitive and colistin-resistant strains were XDR and showed identical ST78 genotype. At variance with prior reports on colistin-resistant strains of A. baumannii, resistance to colistin due to P233S mutation in PmrB sensor kinase did not associate with any measurable reduction in strain fitness, growth characteristics, and virulence.

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