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In this study, we investigated colistin resistance mechanisms associated with the regulation of the pbgP operon in Klebsiella pneumoniae, using four isogenic pairs of colistin-susceptible strains and their colistin-resistant derivatives and two colistin-resistant clinical isolates. Amino acid sequence alterations of PhoPQ, PmrAB, and MgrB were investigated, and mRNA expression levels of phoQ, pmrB, pmrD, and pbgP were measured using quantitative real-time PCR. The phoQ and pmrB genes were deleted from two colistin-resistant derivatives, 134R and 063R. We found that phoQ, pmrD, and pbgP were significantly upregulated in all colistin-resistant derivatives. However, pmrB was significantly upregulated in only two colistin-resistant derivatives and one clinical strain. pmrB was not overexpressed in the other strains. The minimum inhibitory concentration of colistin was drastically lower in both phoQ- and pmrB-deleted mutants from a colistin-resistant derivative (134R) that was overexpressing phoQ and pmrB. However, colistin susceptibility was restored only in a phoQ-deleted mutant from a colistin-resistant derivative (063R) without overexpression of pmrB. In conclusion, two different regulations of the pbgP operon may associate with the development of colistin-resisant K. pneumoniae.

In this study, we investigated the effects of colistin resistance on virulence and fitness in hypermucoviscous (HV) Klebsiella pneumoniae sequence type 23 (ST23) strains. Colistin-resistant mutants were developed from three colistin-susceptible HV K. pneumoniae ST23 strains. The lipid A structures of strains were analyzed by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. Changes in HV were investigated using the string test, and extracellular polysaccharide production was quantified. The expression levels of the phoQ, pmrD, pmrB, pbgP, magA, and p-rmpA2 genes, serum resistance, and biofilm-forming activity were determined. The fitness of colistin-resistant mutants compared to that of the parental strains was examined by determining the competitive index (CI). The colistin-resistant mutants exhibited reduced HV, which was accompanied by decreased formation of capsular polysaccharides (CPS) and reduced expression of genes (magA and p-rmpA2). While there was enhanced expression of pmrD and pbgP in all colistin-resistant derivatives, there were differences in the expression levels of phoQ and pmrB between strains. MALDI-TOF analysis detected the addition of aminoarabinose or palmitate to the lipid A moiety of lipopolysaccharide in the colistin-resistant derivatives. In addition, survival rates in the presence of normal human serum were decreased in the mutant strains, and CI values (0.01 to 0.19) indicated significant fitness defects in the colistin-resistant derivatives compared to the respective parental strains. In hypervirulent HV K. pneumoniae strains, the acquisition of colistin resistance was accompanied by reduced CPS production, impaired virulence, and a significant fitness cost., (Copyright © 2015, American Society for Microbiology. All Rights Reserved.)

Colistin-resistant mutants were obtained from 17 colistin-susceptible strains of Acinetobacter baumannii, Pseudomonas aeruginosa, Klebsiella pneumoniae, and Escherichia coli. The stability of colistin resistance in these mutants was investigated. Three of four colistin-resistant P. aeruginosa mutants recovered colistin susceptibility in colistin-free medium; however, colistin-susceptible revertants were obtained from only one strain each of A. baumannii and E. coli. No susceptible revertants were obtained from K. pneumoniae mutants., (Copyright © 2015, American Society for Microbiology. All Rights Reserved.)

Objectives: The purpose of this study was to investigate variations in IncF plasmids and the genetic environments of bla(CTX-M-15) in CTX-M-15-producing Escherichia coli and Klebsiella pneumoniae isolates from South Korea., Methods: A total of 56 E. coli and 15 K. pneumoniae isolates, which were previously characterized for CTX-M-15 production, sequence type by multilocus sequence typing and replicon type, were included in this study. Replicon sequence typing for IncF plasmids was performed and the genetic environments of bla(CTX-M-15) were determined using PCR and sequencing., Results: A total of 34 and 10 IncF-replicon sequence types (RSTs) were identified among the E. coli and K. pneumoniae isolates, respectively. Only eight and four IncF-RSTs were found in multiple isolates of E. coli and K. pneumoniae, respectively. No common IncF-RSTs were found between E. coli and K. pneumoniae isolates. Five and three different bla(CTX-M-15) genetic environments were identified in E. coli and K. pneumoniae isolates, respectively. Even in the same E. coli clone, diverse IncF-RSTs and bla(CTX-M-15) genetic environments were identified., Conclusions: Diverse IncF plasmids have incorporated into diverse strains of E. coli and K. pneumoniae, contributing to the spread of the CTX-M-15 extended-spectrum β-lactamase in South Korea. It can also be inferred that bla(CTX-M-15) has not been transferred directly from E. coli to K. pneumoniae.