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Genetics of fitness-compensation in experimentally evolved marR mutants of Escherichia coli.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Biochemistry and Microbiology. (Diarmaid Hughes)
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Biochemistry and Microbiology. (Diarmaid Hughes)
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Biochemistry and Microbiology.
(English)Manuscript (preprint) (Other academic)
National Category
Natural Sciences
Research subject
Microbiology
Identifiers
URN: urn:nbn:se:uu:diva-320578OAI: oai:DiVA.org:uu-320578DiVA, id: diva2:1089971
Available from: 2017-04-21 Created: 2017-04-21 Last updated: 2017-04-22
In thesis
1. Constraints on up-regulation of drug efflux in the evolution of ciprofloxacin resistance
Open this publication in new window or tab >>Constraints on up-regulation of drug efflux in the evolution of ciprofloxacin resistance
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The crucial role of antibiotics in modern medicine, in curing infections and enabling advanced medical procedures, is being threatened by the increasing frequency of resistant bacteria. Better understanding of the forces selecting resistance mutations could help develop strategies to optimize the use of antibiotics and slow the spread of resistance.

Resistance to ciprofloxacin, a clinically important antibiotic, almost always involves target mutations in DNA gyrase and Topoisomerase IV. Because ciprofloxacin is a substrate of the AcrAB-TolC efflux pump, mutations causing pump up-regulation are also common.

Studying the role of efflux pump-regulatory mutations in the development of ciprofloxacin resistance, we found a strong bias against gene-inactivating mutations in marR and acrR in clinical isolates. MIC and fitness measurements revealed that amino acid substitutions conferred smaller susceptibility reductions and smaller fitness costs than gene-inactivating mutations, suggesting that resistance mutations in clinical isolates are selected for high fitness rather than high resistance (Paper I and II).

We asked whether the high fitness costs of marR-inactivating mutations could be ameliorated without affecting the resistance phenotype. Multiple independent lineages were experimentally evolved to select for improved growth fitness. Whole genome sequencing revealed mutations affecting marA, lon and arcA as potential compensatory pathways. For the marA and lon mutations the improved growth rate was associated with an increased susceptibility (arcA is being investigated). (Paper III).

An evolution experiment selecting for ciprofloxacin resistance revealed upon whole genome sequencing the expected mutations in drug target and efflux-regulatory genes, but also in genes encoding aminoacyl-tRNA synthetases. We investigated two independently selected leuS mutations, and concluded that they contributed to ciprofloxacin resistance by activating the stringent response that in turn caused up-regulation of genes involved in efflux. However, these leuS mutations incur a high fitness cost (Paper IV).

To summarize, the research findings in this thesis suggest that the potential ciprofloxacin resistome may include more genes than previously thought, but a strong selection for high fitness selectively purifies many resistance mutations from clinical isolates. In conclusion, selection for high relative fitness constrains the spectrum of mutations that survive and get fixed in clinical populations of bacteria.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 48
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 1335
Keywords
ciprofloxacin, antibiotic resistance, drug efflux, bacterial fitness, marR, acrR, marA, experimental evolution
National Category
Natural Sciences Medical and Health Sciences
Research subject
Microbiology
Identifiers
urn:nbn:se:uu:diva-320580 (URN)978-91-554-9923-5 (ISBN)
Public defence
2017-06-09, B22, BMC, Husargatan 3, Uppsala, 09:00 (English)
Opponent
Supervisors
Available from: 2017-05-17 Created: 2017-04-22 Last updated: 2017-06-07

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