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Klebsiella pneumoniae is an opportunistic pathogen that is often difficult to treat due to its multidrug resistance (MDR). We have previously shown thatK. pneumoniae strains are able to “adapt” (become more resistant) to the widely used bisbiguanide antiseptic chlorhexidine. Here, we investigated the mechanisms responsible for and the phenotypic consequences of chlorhexidine adaptation, with particular reference to antibiotic cross-resistance. In five of six strains, adaptation to chlorhexidine also led to resistance to the last-resort antibiotic colistin. Here, we show that chlorhexidine adaptation is associated with mutations in the two-component regulatorphoPQ and a putative Tet repressor gene (smvR ) adjacent to the major facilitator superfamily (MFS) efflux pump gene,smvA . Upregulation ofsmvA (10- to 27-fold) was confirmed insmvR mutant strains, and this effect and the associated phenotype were suppressed when a wild-type copy ofsmvR was introduced on plasmid pACYC. Upregulation ofphoPQ (5- to 15-fold) andphoPQ -regulated genes,pmrD (6- to 19-fold) andpmrK (18- to 64-fold), was confirmed inphoPQ mutant strains. In contrast, adaptation ofK. pneumoniae to colistin did not result in increased chlorhexidine resistance despite the presence of mutations inphoQ and elevatedphoPQ ,pmrD , andpmrK transcript levels. Insertion of a plasmid containingphoPQ from chlorhexidine-adapted strains into wild-typeK. pneumoniae resulted in elevated expression levels ofphoPQ ,pmrD , andpmrK and increased resistance to colistin, but not chlorhexidine. The potential risk of colistin resistance emerging inK. pneumoniae as a consequence of exposure to chlorhexidine has important clinical implications for infection prevention procedures.

antimicrobial agents and chemotherapy

Mechanisms of Increased Resistance to Chlorhexidine and Cross-Resistance to Colistin following Exposure of Klebsiella pneumoniae Clinical Isolates to Chlorhexidine