Emergence of pmrB Mutation-Mediated Colistin Resistance in Clinical Acinetobacter baumannii.

OBJECTIVES: While colistin-resistant Acinetobacter baumannii continues to be discovered, this study aimed to investigate the molecular mechanism of colistin resistance in two clinical Acinetobacter baumannii strains (coli-S and coli-R) isolated from a single patient. METHODS: Antimicrobial drug susceptibility was assessed by broth microdilution method. We monitored growth curves, and analyzed biofilm formation using confocal laser scanning microscopy. Combined with MLST, next generation and third-generation sequencing techniques, the genomic differences of the strains were compared, and expression of mutant genes was detected by RT-qPCR. Meanwhile, SWISS-MODEL was used to analyze the three-dimensional structure of PmrB. RESULTS: Coli-R was isolated in one patient who was treated with colistin for 7 days. The MICs of coli-S and coli-R were 1 μg/mL and >32 μg/mL respectively. Both isolates were typed as ST2. Compared with coli-S, coli-R exhibited slower growth rate, lower stationary-phase cell density, and higher biofilm formation ability. Whole genome sequencing showed that genetic variations between the strains were located in pmrB, filE and bap. In coli-R, there was a single deletion mutation in pmrB (ΔAla32-Glu35), which was associated with the structural change of PmrB and activation of the PmrAB two-component system. CONCLUSIONS: Our study demonstrated that the deletion mutations in pmrB and bap had evolved in Acinetobacter baumannii within host. The genetic variations were associated with colistin resistance, primarily manifested as the activation of the PmrAB two-component system and biofilm formation.