Alteration in the Morphological and Transcriptomic Profiles of Acinetobacter baumannii after Exposure to Colistin

<i>Acinetobacter baumannii</i> is often highly resistant to multiple antimicrobials, posing a risk of treatment failure, and colistin is a "last resort" for treatment of the bacterial infection. However, colistin resistance is easily developed when the bacteria are exposed to the drug, and a comprehensive analysis of colistin-mediated changes in colistin-susceptible and -resistant <i>A. baumannii</i> is needed. In this study, using an isogenic pair of colistin-susceptible and -resistant <i>A. baumannii</i> isolates, alterations in morphologic and transcriptomic characteristics associated with colistin resistance were revealed. Whole-genome sequencing showed that the resistant isolate harbored a PmrB_L208F mutation conferring colistin resistance, and all other single-nucleotide alterations were located in intergenic regions. Using scanning electron microscopy, it was determined that the colistin-resistant mutant had a shorter cell length than the parental isolate, and filamented cells were found when both isolates were exposed to the inhibitory concentration of colistin. When the isolates were treated with inhibitory concentrations of colistin, more than 80% of the genes were upregulated, including genes associated with antioxidative stress response pathways. The results elucidate the morphological difference between the colistin-susceptible and -resistant isolates and different colistin-mediated responses in <i>A. baumannii</i> isolates depending on their susceptibility to this drug.