Mechanisms of increased resistance to chlorhexidine and cross-resistance to colistin following exposure of Klebsiella pneumoniae clinical isolates to chlorhexidine

ABSTRACT

Klebsiella pneumoniae is an opportunistic pathogen which is often difficult to treat due to its multidrug resistance (MDR). We have previously shown that K. pneumoniae strains are able to “adapt to” (become more resistant) to the widely used bisbiguanide antiseptic chlorhexidine. Here we investigated the mechanisms responsible and the phenotypic consequences for 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 regulator phoPQ and a putative tet-repressor gene (smvR), adjacent to the MFS family efflux pump smvA. Up-regulation of smvA (10-27 fold) was confirmed in smvR mutant strains and this effect and the associated phenotype was suppressed when a wild type copy of smvR was introduced on plasmid pACYC. Up-regulation of phoPQ (5-15 fold) and phoPQ-regulated genes, pmrD (6-19 fold) and pmrK (18-64 fold), were confirmed in phoPQ mutant strains. In contrast, adaptation of K. pneumoniae to colistin did not result in increased chlorhexidine resistance despite the presence of mutations in phoQ and elevated phoPQ, pmrD and pmrK transcript levels. Insertion of a plasmid containing phoPQ from chlorhexidine adapted strains into wild-type K. pneumoniae resulted in elevated expression levels of phoPQ, pmrD and pmrK and increased resistance to colistin but not chlorhexidine. The potential risk of colistin resistance emerging in K. pneumoniae as a consequence of exposure to chlorhexidine has important clinical implications in infection prevention procedures.