Copy Number of an Antibiotic Resistance Locus Controls Aminoglycoside Heteroresistance and a Virulence Switch in Acinetobacter baumannii Open Access
Anderson, Sarah (Fall 2019)
Published
Abstract
Acinetobacter baumannii is a Gram-negative opportunistic pathogen. This bacterium is a significant public health threat, in part due to the high levels of antibiotic resistance exhibited by many strains of this pathogen. Therefore, more research is needed into the identification of novel A. baumannii drug targets, as well as a more complete understanding of the antibiotic resistance mechanisms employed by this species. This work is focused on aminoglycoside heteroresistance and colony opacity variation in the multidrug-resistant A. baumannii strain AB5075. Heteroresistance is a phenomenon wherein a subpopulation of cells within an isogenic bacterial strain exhibit increased resistance to an antimicrobial. Colony opacity variation refers to the production of opaque and translucent colony types by a bacterium; in A. baumannii this involves a high-frequency switch between virulent opaque (VIR-O) and avirulent translucent (AV-T) colonies. Here, we establish that both aminoglycoside heteroresistance and opacity switching are controlled by copy number of a horizontally-acquired antibiotic resistance locus. AB5075 spontaneously gives rise to cells with RecA-dependent high-copy amplifications of this locus, which includes the aminoglycoside resistance gene aadB. This leads to increased aadB expression in a subset of cells, which results in heteroresistance to tobramycin and gentamicin. Variations in copy number of this locus also result in changes in the frequency of VIR-O to AV-T switching in AB5075. Opaque cells exhibiting a single copy of this locus switch to AV-T at a low frequency, a phenotype designated as low-switching opaque (LSO). Duplications of this locus lead to nearly three-log fold increases in switching to AV-T. The element responsible for controlling switching is encoded at the 5’ end of the aadB gene, and our evidence suggests that this element is an sRNA. We further determined that LSO variants exhibit decreased virulence relative to normal VIR-O cells carrying the duplication, possibly due to decreased expression of virulence genes in the LSO variant. This work establishes the important role of gene duplications and amplifications in mediating phenotypic heterogeneity in A. baumannii. This research also increases our understanding of a complex virulence switch, which could eventually lead to the development of novel virulence-targeted therapeutics for this pathogen.
Table of Contents
Abstract
Acknowledgements
Table of Contents
List of Tables and Figures
Chapter 1: Introduction……………………………………………………………………………1
Chapter 2: Aminoglycoside Heteroresistance in Acinetobacter baumannii AB5075…………...38
Chapter 3: Copy Number of an Integron-Encoded Antibiotic Resistance Locus Regulates a Virulence and Opacity Switch in Acinetobacter baumannii AB5075…………………………...81
Chapter 4: Distinguishing Colony Opacity Variants and Measuring Opacity Variation in Acinetobacter baumannii……………………………………………………………………….153
Chapter 5: Discussion…………………………………………………………………………..166
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Copy Number of an Antibiotic Resistance Locus Controls Aminoglycoside Heteroresistance and a Virulence Switch in Acinetobacter baumannii () | 2019-11-06 10:59:35 -0500 |
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