Unstable tandem gene amplification generates heteroresistance (variation in resistance within a population) to colistin in Salmonella enterica. Issue 2 (2nd August 2016)

• Record Type: Journal Article
• Authors: Hjort, Karin; Nicoloff, Hervé; Andersson, Dan I
• Is Part Of: Molecular microbiology. Volume 102:Issue 2(2016)
• Page Start: 274
• Page End: 289
• Publication Date: 2016-08-02
• Subjects: Molecular microbiology -- Periodicals; 572.829
• DOI: 10.1111/mmi.13459
• Languages: English
• ISSNs: 0950-382X

Abstract

Summary: Heteroresistance, a phenomenon where subpopulations of a bacterial isolate exhibit different susceptibilities to an antibiotic, is a growing clinical problem where the underlying genetic mechanisms in most cases remain unknown. We isolated colistin resistant mutants in Escherichia coli and Salmonella enterica serovar Typhimurium at different concentrations of colistin. Genetic analysis showed that genetically stable pmrAB point mutations were responsible for colistin resistance during selection at high drug concentrations for both species and at low concentrations for E. coli . In contrast, for S . Typhimurium mutants selected at low colistin concentrations, amplification of different large chromosomal regions conferred a heteroresistant phenotype. All amplifications included the pmrD gene, which encodes a positive regulator that up‐regulates proteins that modify lipid A, and as a result increase colistin resistance. Inactivation and over‐expression of the pmrD gene prevented and conferred resistance, respectively, demonstrating that the PmrD protein is required and sufficient to confer resistance. The heteroresistance phenotype is explained by the variable gene dosage of pmrD in a population, where sub‐populations with different copy number of the pmrD gene show different levels of colistin resistance. We propose that variability in gene copy number of resistance genes can explain the heteroresistance observed in clinically isolated pathogenic bacteria. Abstract : Heteroresistance, a phenomenon where subpopulations of a bacterial isolate exhibit different susceptibilities to an antibiotic, is a growing clinical problem. Here we show that gene amplification of large chromosomal regions containing resistance genes confers a colistin heteroresistant phenotype. We propose that variability in gene copy number of resistance genes is a common explanation for the heteroresistance observed in clinically isolated bacteria.