Loss of Hypermucoviscosity and Increased Fitness Cost in Colistin-Resistant Klebsiella pneumoniae Sequence Type 23 Strains

Abstract

ABSTRACTIn this study, we investigated the effects of colistin resistance on virulence and fitness in hypermucoviscous (HV)Klebsiella pneumoniaesequence type 23 (ST23) strains. Colistin-resistant mutants were developed from three colistin-susceptible HVK. pneumoniaeST23 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 thephoQ,pmrD,pmrB,pbgP,magA, and p-rmpA2genes, 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 (magAand p-rmpA2). While there was enhanced expression ofpmrDandpbgPin all colistin-resistant derivatives, there were differences in the expression levels ofphoQandpmrBbetween 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 HVK. pneumoniaestrains, the acquisition of colistin resistance was accompanied by reduced CPS production, impaired virulence, and a significant fitness cost.